In general there are three kinds of work commitments involved in a team.
The first is Maintainership, where an individual’s participation is driven by what the team needs. This typically involves things that are “duties” as opposed to other contributions, things that must happen for the project to continue functioning. This includes regular reviews, leadership work, project management, regular triage, etc. This may include feature work as well, depending on what the team has decided as its priorities. People who are maintainers can and likely will be contributing to the team in the ways below as well.
The second is Contributorship, where an individual’s participation is driven by what they wish to do. This can involve occasional feature work and even reviews, but is not necessarily stuff that would be considered a team’s “duties”, and may in fact end up producing more of a burden on maintainers in terms of reviews and project management. Their contribution is still valued, however it is not stuff that must happen for the team to keep functioning.
The third is Advisory. These are high context individuals who are around to be consulted when necessary. Their contribution is largely driven by being asked to weigh in on topics. Examples of this are groups of ecosystem maintainers who may have relevant opinions on a topic, or groups of historical maintainers of a codebase who have a lot of context but are not typically active participants in review.
Teams and project groups may be defined to be entirely consisting of people of one kind of category, or consist of a mix. A large team may create a separate “advisory group” subteam, but a smaller team may include the advisors on the team itself.