A doypack and a flat bottom pouch mainly differ in their construction and the way they stand.
A stand-up pouch, also known as a doypack, has a bottom gusset that opens up when the pouch is filled. This allows the pouch to stand upright on its own. A flat bottom pouch has a completely flat base and side gussets. This generally makes the packaging more stable and gives it a more box-like shape.
Thanks to its flat front, back and side panels, a flat bottom pouch also offers plenty of space for branding.