Hi! Applying to be a Page Artist means you're interested in creating a fully rendered page, (which is 2480x3508px!) complete with a background.
Applying as a Spot Artist means you're interested in creating a small piece of art that compliments a writer's work, be it a scene from the story, a Polaroid of the characters, etc. Think of a photo as part of a story in a newspaper! These typically do not take up a full page. Though, if you're willing and able, you may create a full page art for the writer's story. Hope this answers your question!