The Tech Tip Converting to version 11 more than once covers general usage of the catalog.xml file and how it can be used to manage the conversion process. In short, if you have a structure and data file you convert from a previous version (4D 2004 or earlier) to 4D v11 SQL or 4D v12, then the catalog.xml file is generated as part of conversion. If in the future you want to open said data file with another conversion of the structure file, then you must use the catalog.xml when re-converting the old structure, otherwise the data file and structure file's UUID's will not match.

In the same way that developers can use this to convert one old structure multiple times, so that each new structure has the same UUID, two different structures can be converted with the same catalog.xml so that they share a UUID. This can be very valuable if in previous versions you had two 4D structures with different code bases but matching table schemas, and used the different structures to access the same data file. In 4D v12 this isn't possible unless the two structures share a UUID. The catalog.xml file can be used to ensure that both structures have the same UUID and can open the same data file.