When a stylist works on a photoshoot, he or she basically chooses the clothing, the accessories, and/or the general “look” of the model for the shoot. In the case of a magazine shoot, the stylist would be given some direction from the magazine or art director as to what the theme was (spring summer bright colours, animal prints, etc…) and what brands need to be featured.
Remember that the brands featured in a glossy magazine are mostly the brands that advertise, so the magazine needs to make sure they get editorial coverage. The team would be named (photographer, hair, make up, etc…) and sometimes the stylist would have some say in this. Once the “concept” for the shoot was finalized with the photographer, the stylist must then pull the clothing for the shoot.