The RectTransform component defines what space a slot with UIX components on it will use, based on the total amount of space that has been provided. This is mostly used to encapsulate elements with inside it, then provides options to anchor, offset, and pivot the placement of the RectTransform container.
The RectTransformComputedProperties component is useful when using this component as a reference, returning you the size of the actual rect transform for your calculations.