A board is restricted to the users by shares of the filter on which the board is defined. In your case the board can be used by users who have access to mentioned projects.
You can read about restricting access to board here:
If you do not want the users of the projects use the board or the filter then do not share the filter with them
I’m a designer on the Jira team. For a long time, I’ve fielded questions from other designers about how they should be using Jira Software with their design team. I’ve also heard feedback from other ...
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