A design that looks right on your phone can still fail on fabric. The canvas checks one specific thing well: whether your art fits, scales, and reads inside the real print area on the real garment color. It does not test ink-on-fabric color accuracy, hand-feel, or wash durability — no screen can do that. Keep that distinction in mind for everything below.
The dashed rectangle you see on a blank garment is not decoration — it is the literal print area. A face, a wordmark, a line of small type: anything that has to survive the print has to live inside that box, not just inside the visible outline of the shirt.