Resizable indicates that you first place the brick, then select a face, edge, or vertex to increase/decrease the volume of said brick, but a region selection implies that you select two vertices of a prismatic region, though each vertex could be changed when needed.
Roblox uses resizable bricks, where the faces can be extruded out in linear directions, whereas World Edit uses the wood axe to select two voxels, and all blocks inbetween the coordinates for these selected voxels would be included in the selection.