Document the Plan object model

What product or feature(s) affected?

Planning/Issue Tracker
What docs or doc section affected? Include links or paths.

https://docs.gitlab.com/ee/user/project/issues/
Is there a problem with a specific document, or a feature/process that's not addressed sufficiently in docs?

The relationship between Epics, Issue, Labels, Groups, and Projects is pretty hard to understand. The GitLab Marketing team needed to do a workshop to teach people what this relationship was so they could use the issue tracker. see https://gitlab.com/gitlab-com/marketing/marketing-operations/issues/975

Any concepts, procedures, reference info we could add to make it easier to successfully use GitLab?

see above
Include use cases, benefits, and/or goals for this work.
- onboarding
- help users understand how to use the issue tracker faster
If adding content: What audience is it intended for? (What roles and scenarios?)

GitLab Users. Almost anyone can benefit from project management. The docs should describe how to use the issue tracker in a way that makes sense to sales and marketing people in addition to software developers.

I recommend a section describe how all the elements interplace along with a visual diagram similar to this:

credit to @jjcordz for making this