Board Thread:Warhammer 40k General Discussion/@comment-14745711-20150626121156/@comment-26436507-20150626162210

The various litrature available mentions a range from robes to combats 'denims' and body gloves. I'd very much imagine it has to do with the requirements on them during the time 'out of theater' e.g. refit & repair, specific drill, actual down time, religious/social occasion/ritual etc

I imagine that the chapters leadership would have some say as would the environment.

As Space Marines are effective a RRF (Rapid Reaction Force) lots of times then they will also have various states if equipment to deal with the various states around this e.g. body shell and fatigues/denims for 2minute standby or slacks for gym sessions and PJ'S for bed (lol)