The difference between a catalog and booklet envelope is that the flap and opening of a catalog envelope is on the shorter side of the envelope whereas a booklet envelop has the flap and opening on the longer side.
For most mailings we recommend using booklet envelopes as stuffing the envelopes is much more efficient. Catalog envelopes are used for aesthetic purposes as some mailers prefer the look and presentation of a catalog envelope as well as the user experience of pulling documents from the top of the envelope as opposed the the side.
The same catalog and booklet terminology is used for all types of envelopes including 9x12, 6x9, etc, but catalog envelopes should always have their flap on the shorter side of the envelope whereas the flap on the booklet envelopes are on the longer side of the envelope.
For instance for a 9x12 envelope the flap and opening will be on the 9 inch side for catalog envelopes and for booklet envelopes the flap and opening will be on the 12 inch side.