Hi,
We are using the following scenarios:
1. All internal catalogues (negociated by purchasing) are stored in MDM and then called to the SRM Portal via an web call structure. This requiers you to to set-up the call structure in SRM and the upload all the products to MDM via an excel upload.
2. For external catalogues that are vendor maintained we are using the Punch Out catalogues. The main difference between this and the catalogues at point one is that you remotly connect to the vendors website via a web call structure and when all shopping is done you just submit the shopping cart via the OCI (open catalogue interface). This though needs to have the vendor the catalogue OCI enabled.