A Rect represents a 2-dimensional rectangle. It consists of a starting x and y coordinate, and a width and length. Most, if not all, components will have these as proportions of the total bounds, between 0 and 1. For example, for the SpriteProvider component, the Rect property bounds the sprite to be rendered, with the values being proportions of the total width and height of the sprite image.
For dynamic variables and other components requiring a type, the type for Rect is Elements.Core.Rect.