The Master Catalogue itemizes and lists all of the different breakdown forms that you have identified, and each time you identify another one it is added to your Master Catalogue in your Celtx file. If you go over to the left hand Project Library you will see your Master Catalogue listed there along with your screenplay and any other section you have added to your project with the Add button up top.
Next to the words Master Catalogue you will see that there is a number listed there. This number is the number of how many entries are in your Master Catalogue, which is the number of specific elements that you identified. Go ahead and select the Master Catalogue and it will open up the Master Catalogue for your script in another Celtx tab.
The Master Catalogue is essentially broken into two views in Celtx. On the top you will have the list of all the items that were identified in the script by name and type. For example, you may have "john's torn shirt" in the name and "wardrobe" in the type. Below that will be the accompanying form for that specific type. In the case of "john's torn shirt" this would be a wardrobe form, as was identified. In that wardrobe form you will not find much information already filled out, except for every scene where "john's torn shirt" was found and identified. In each of those forms the proper information is filled out and then stays associated with that item in the Master Catalogue. In this way the Master Catalogue will act as your directory to every filled out form for every identifiable item in the script. Who actually fills out each form depends on who is doing what in the production, but it is all orchestrated from a centralized location. For example, the location scout will likely fill out the location sheet as well as being in charge of things like the location release.