Crew Changes

I dont quite understand the question? Real life or SimSig timetabling?

Drivers have diagrams which are planned according to contract and fatigue rules. Trains will often change crews where the first driver is either up on his hours before a PNB or is at the extent of his route knowledge. In some cases Drivers can only work so far one way before having a break and working something back within their full booked shift. Subject to the caveat that all companies have different T&Cs and planning rules, Drivers generally need a break every 5 hours of work and can only conduct safety critical work for 11 hours from booking on. Some companies are definetely more restrictive.

Do you mean when is a crew change activity added and why?

Generally speaking crew changes are shown on non passenger services which could depart early to force them to wait until the relief crew arrive and add the curveball when crew are delayed. Passenger services don't generally show crew changes as they're more punctual and adherent to the timetable.