On a screen printing press, the shirt is pulled over the platen (also called a pallet) and held with a light spray or tape adhesive so it cannot move between colors. Platens come in many sizes and shapes: standard, youth, sleeve, leg, pocket, hat and oversized boards for jumbo prints. DTG printers and heat presses use platens of their own.
The platen sets physical limits. A design cannot print larger than the platen and screen allow, small garments may not fit over a large board, and locations such as sleeves, hoods or pant legs need dedicated platens. A zipper, thick seam or pocket lying on the platen creates an uneven print surface.
When planning placements, ask which platen will be used for each size and location, and whether a big front print will be one size across the whole run or scaled by garment size. See jumbo prints and sleeve, hood and pocket prints.