First of all, we know that a rectangular prism is a solid. Therefore slicing it with a plane, (or a flat shape) basically divides it into two rectangular solids.
This is particularly so if the plane cuts or slices the prism from the horizontal.
Also, note that if the plane cuts from the vertical, (that is from top to bottom) you might most probably end up with two square prisms or solids.
However, since no specific dimensions are given, then slicing a rectangular prism through with a plane would give you a rectangular cross section. The cross section is actually the shape on the inside of either half.
When a plane slices a rectangular prism from the horizontal, the cross section is a rectangle.
When it slices a a rectangular prism from the vertical, the cross section is a square.
When it slices a rectangular prism from one vertex to another from across opposite sides, the cross section is a right angled triangle.