The so-called subordinate universes don't mean that this Crystal Wall universe is right next to the Toriel Multiverse, nor does it mean that there is a connection channel between the two universes that qualifies them as a subordinate universe. Rather, it refers to two Crystal Wall universes that share the same rules and power systems, but one is stronger and the other weaker, creating a primary-secondary relationship.
This situation is not uncommon in the Chaotic Void Sea. After all, there are countless large and small Crystal Wall universes within the Chaotic Void Sea, but there can't be that many different fundamental rules and power systems. Thus, some Crystal Wall universes with extremely similar basement rules and power systems are grouped into the same system.
