We create the catalog in ACC based on the catalog design / repository xml.
Basically, productcatalog xml is the repository definition file, in which you define how the underlying data would store in your DB by mentioning tables, relations, constraints etc., ACC uses this xml to display the options while you create the catalog.
So, the steps would be as follows.
1. Finalize your catalog structure according to your business requirements.
2. Design the repository / tables.
3. Create catalog elements via ACC.
Hope this helps.
Keep posting any questions you still may have.