Project staff assignments are created by (outputs from) the PMI process to acquire project team when staff is assigned to the project. Related documents may be a team directory, listing, or organization chart. As new people join, there onboarding activities such as sending out a memo introducing the new staff and their roles. Then the assignments are used as input to the develop project team, manage project team, and develop schedule processes.
The project staff assignments are described in section 22.214.171.124 of the PMBOK 5th edition.