I may have been wrong I use ced and ced+ very rarely but you can define set of tiles in those xml files. Other thing is that ced reads the client files and after reading it caches them. So to clarify the sets or groups of tiles like whole structures, multis, ships etc. are in those xml. And static/terrain tiles are just read as they are from uo files but only if there is no cache generated already.
so you may want to clear the cache. Button is available on the main login screen.
Cache files are kept in c:\Users\<windowsUsername>\AppData\Local\CentrED-plus\Profiles\<nameOfProfile>\ and have .cache extension