"Done" - some clarification needed

You and your team need to work together to craft and agree on a Definition of Done. This takes out the guesswork of what Done actually means. This should not drastically change each sprint and it really should be something that applies to all stories and tasks. If you have specific changes for each story, that's where the AC comes in.

Do a google search for Definition of Done template and it will help give some ideas. 

1. When (which event) and by whom (from the Scrum team) is "done" crafted/formulated?

From the Scrum Guide section describing the Sprint Retrospective (emphasis added by me)

During each Sprint Retrospective, the Scrum Team plans ways to increase product quality by improving work processes or adapting the definition of "Done", if appropriate and not in conflict with product or organizational standards.

That indicates where the DoD is reviewed.  How is it originally defined is not really mentioned.  The practice I coach is to define one the very Sprint Planning.  It doesn't have to be perfect because we will review it at every Retrospective as needed.  There can also be an organizational DoD. I coach that any organizational DoD be formed at the time that Scrum is determined to be the framework used and before any teams are formed.  If one is determined to be needed after the fact, it should be crafted as soon as that need is determined.  Then every team will need to review their DoD at their next retrospective and adapt to ensure that the team DoD does not attempt to subvert that organizational DoD. 

2. For each item there are AC (acceptance criteria) which, if fullfilled are resulting also in a useable and potentially releasable increment. BUT as far as I understand AC, same as the scope, are renegotiatable and flexible AND each item has different AC. Whereas "done" is a fixed thing and should be valid for ALL the items in the selected sprint/product? backlog (also not clear if "done" is valid for the SPRINT or for the PRODUCT backlog???)?

The DoD is for the entire increment being produced in the Sprint.  ACs are best practices adopted as part of the User Story.  They apply to the individual story and convey how the Scrum Team will know that the User Story is satisfied.  Yes, they can be negotiable and flexible but does it really make sense to change them when trying to determine if the story is finished? In my experience that just leads to problems that affect the actual increments being built and is a symptom of poor refinement practices.  The ACs should be "firm" when the Development Team decides that the story is "ready" to be pulled into a Sprint.  All of the negotiation should have occurred prior to that.

3. Is "done" different for each sprint or is it fixed for the whole bunch of sprints planned in a scrum project?

From the Scrum Guide section that defines the DoD (emphasis added by me)

When a Product Backlog item or an Increment is described as "Done", everyone must understand what "Done" means. 

This is the definition of "Done" for the Scrum Team and is used to assess when work is complete on the product Increment.

DoD is used by "everyone" to have a shared understanding of when an increment is complete.  "Everyone" means just that. It is for Development Team, Product Owner, Scrum Master, Stakeholders, Management.  Everyone that has a vested interest should understand it.  It is applied a product increment.  Since the result of every Sprint is a potentially releasable product increment it is applied individually at every Sprint.  I showed earlier how the DoD can be adjusted at every Retrospective so you can assume that the DoD can vary from Sprint to Sprint as long as it is clear for "everyone" to understand and is well communicated to "everyone". 

There are some Scrum organizations that say that the DoD should be applied for every story but the Scrum is pretty clear that is to be applied to the increment.  I will admit that I have had teams that applied the DoD to each story. I did not really object because their DoD was "generic" enough that it could apply at that level (mostly around code reviews, running in product environments, defects addressed, etc).  But I did make it clear to them how the DoD was defined to be applied and that since we used AC in our stories, that was the primary measure of whether a story was complete. 

And by the way, in case you don't have a link to the Scrum Guide I'll leave it here.  https://www.scrumguides.org/scrum-guide.html.  If English is not your native language you can find many language translations at this url. https://www.scrumguides.org/download.html

Hope this helps. 

1. When (which event) and by whom (from the Scrum team) is "done" crafted/formulated?

A Development Team should understand what “Done” means no later than Sprint Planning, otherwise they can’t really anticipate what work would be involved, or forecast how much of it they can take on.

2. For each item there are AC (acceptance criteria) which, if fullfilled are resulting also in a useable and potentially releasable increment. BUT as far as I understand AC, same as the scope, are renegotiatable and flexible AND each item has different AC. Whereas "done" is a fixed thing and should be valid for ALL the items in the selected sprint/product? backlog (also not clear if "done" is valid for the SPRINT or for the PRODUCT backlog???)?

Perhaps it is best to think of acceptance criteria as representing a certain level of “Done”.

3. Is "done" different for each sprint or is it fixed for the whole bunch of sprints planned in a scrum project?

There ought to be a Definition of Done which is appropriate for the latest product increment being developed. The Definition of Done may be improved, by the teams doing the work, at any time.

1. DOD is defined in Sprint retrospective meetings where development team refines/decide what is definition of done.

2. I will give you an example of DOD which my team is using:

    2.1 the task/story/subtask should have an automation test case( automation was recently introduced in the team)

    2.2 Google analytics events are implemented and tested 

     2.3 Performance testing done and results are inside the acceptable limit.

Above are some some of the standard DOD which remains same in all the sprints. The acceptance criteria may differ from DOD and this depends on team to team.  

The Acceptance Criteria (AC) is unique to each story. Similar stories would likely have similar AC, but that doesn't mean the AC is the same for all. AC applies at story level (product backlog item).

The Definition of Done (DOD), on the other hand, applies at the increment level (that is the sum of all work completed in a sprint). It (the DOD) can be adapted as the team sees fit.

Think about the AC and DOD as follows.

Construction project.

Acceptance Criteria (unique):

• Foundation: dig at X feet; use Z concrete type
• Structure: use steel frames type W
• Windows (1st floor): use brown UPVC size S1
• Windwos (2nd floor): use brown UPVC size S2
• Doors: ...
• ...

Definition of Done:

• Activities to be performed in agreement with City Hall building permit no. ## and associated guidelines
• Health & Safety Inspection to be performed daily once project starts
• QA Engineers to verify foundation, structure, etc and blueprints match
• New building to pass all construction and fire safety inspections
• ...

Not the perfect example, but should help you understand.

Hope it's much clearer now.

@Eugene M-

Thanks for sharing such a good example of AC and DOD.

I was able to give  an example of DOD but couldn't think of any good example of AC. I am noting down your examples  for future purpose :)