Typically any type or size of creative content counts as a fill -- writing, art, crafts, photography, etc.

Most of the time, it's one fill to one prompt/square. However, some people like to use multiple prompts for the sections in a longer work, like a chaptered story. I've also had prompters ask to combine 2+ squares in my Poetry Fishbowl. Some fests have meta prompts that do things like combine 2 other prompts. So if you've got a good idea, go with it. This is a low-pressure, high-inspiration community.