The reflections are entirely other worlds, like how we went to the First in Shadowbringers which is one of the 13 reflections. Seven of them have been reabsorbed back int the Source, and the 13th shard we've been to as well (alebit briefly) in the Crystal Tower raids as the World of Darkness, meaning there are 4 worlds we've never seen.

The rejoinings basically are that the reflection gets destroyed entirely while a devastating cataclysm impacts the Source, usually resulting in massive death and devestation on the world. By doing the rejoinings, the Ascians were attempting to weaken the bindings on Zodiark so that he could be freed. Fandaniel released him prematurely in main story, which is why if you look at him in the trial, actual pieces of his being are also missing.