What does different LOV means while adding a participant as a worker or manager

Hi Experts,

In redwood, for a performance document when we add a participant either by Manger/Employee we get a dropdown items for "Select Person from" field, which has values in some cases and no values in some cases, less values in some other cases.

Could you please explain what does different dropdown value means while selecting "Select Person from" field.

Thanks

Mohit Srivastava