Currently the role descriptions give a general idea as to who can add what, but doesn't speak to who can delete certain things within the Aha! instance. You might be indicating that by saying (i.e.) that the Contributor role can *manage*".. strategy, releases, and ideas" meaning that they can both create and delete, but I'm not sure. Please consider adding that text to the appropriate user roles.
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That's correct that for the Contributor role - when we indicate that this user type can "Manage," this means that they can both create and delete. This is a similar to the product owner's ability to manage users (create/delete).
At this time, we're unlikely to make updates here in an effort to make things concise but hope that this helps to clarify the permissions.