I have not seen a definition of Unit Participant (UP) yet.
Registration Guidebook, July 2019
Adult program participant. An adult program participant is any person 18 but not yet 21 years of age who registers to participate in a program in which youth members also participate; obligates himself or herself to regularly attend the meetings; fulfills a member’s obligation to the unit; subscribes to the Scout Oath; and participates in an approved program based on the current guide-lines of the Boy Scouts of America. Adult program participants are subject to the same guidelines as adult Scouters when required by policies and guidelines.
INDIVIDUAL JOINING REQUIREMENTS
Youth Application Requirements (p. 22, extract)
The official youth application, paper or online, is always required for registering a new youth.
18 years of age or older who are participating in a youth program, must complete the official adult application including the CBC authorization form, and must complete Youth Protection training.