In practice
It applies to every item. Acceptance criteria describe what 1 piece of work must do. The definition of done describes the standard all work must meet.
It shouldn't be a long process manual. It should be short enough for the team to remember and use.
Why it matters for product builders
Without a shared standard, “done” drifts toward whoever wants to move on first. The missing tests, docs or release work show up later.
What changes in AI-first products
The standard must be written as checkable conditions. An agent can't read an unwritten team norm. If it isn't stated, it isn't part of the build.