There are two types of parts in an Autocad MEP MvPart Catalog. A block based MvPart and a Parametric MvPart. Block based parts are created from 3D solids or Mass Elements. Additional sizes that are added to the part are done by creating a new block and adding it to the catalog drawing file. Parametric parts are created using dimensions and constraints. All sizes are modeled from a single part that is ‘flexed’ by its parameters to create a almost unlimited amount of size combinations. Even though each part type is created using content builder, the interface to do so is decidedly different for a block based part than that for a parametric part. The main advantage of a parametric based part is that it is quite easy to create new custom sizes as long as the model is built correctly.
How do you tell when adding a part if it is parametric or block based? It is quite easy by looking at the icon of the part in the Add Multi-View Parts Dialog. The picture below is worth a thousand words.