Not "case" but CASF

CASF is Canadian Active Service Force, this was Canada's overseas field force created in 1939.

Some general comments on these photos - the CASF in December 1939 consisted of two infantry divisions, both earmarked for the United Kingdom. The 1st began going overseas in December 1939, the 2nd went over in the summer of 1940. Group photos such as the one you describe were extremely common, as individual units posed for photos - generally done by contracting local portrait photographers from civilian studios, as they had the requisite equipment. The photos were done for, I presume, historical posterity, and copies of these photos can be found in many unit archives now, as well as some private collections. I'm not sure how many soldiers at the time would have obtained copies of these photos as 'souvenirs', certainly I can't imagine there being much reason to take them overseas with them.

I liken them to team photographs which are a staple in athletic communities; not just the recording of history but also a means of displaying and fostering pride.

Such photos are still popular in the current Canadian Forces, the example most readily coming to mind being instructional courses, where students and staff (often coming from a variety of different units) will sit for a formal portrait to mark the occasion of their short time together.