If you mean rectangle, yes it is a parallelogram which is a plane figure.
But if you mean rectangular, it must be followed by a noun because rectangular is an adjective.
Take this for example:
Example A: The shape of a lot (a parcel of land) is a rectangle. In a word problem, this is paraphrased as: A rectangular lot has lot area of ____ , or an area of ______, but NEVER a volume or capacity of ________ because it is a plane figure (polygon) - flat, no height, only width and length.
Example B: The shoe box is rectangular. In a word problem, this is paraphrased as: A rectangular box has length, width, and height or ___ cm, _____, cm, ____, respectively. You'll be asked to compute for its Volume and Surface Area. It is 3-dimensional solids (polyhedron), not flat.
Hope this helps.