Star Warfare's online gaming modes appears to have some sort of mechanism which separates players into different servers or groups depending on various factors. As of now, three known factors are known in determining whether players can play with each other or not.

Geographical segregation

Most players within Asia can only play with other Asian players, and are unable to play with players from other parts of the world. Similarly, players outside of Asia can play with other non-Asians, but are unable to see rooms hosted in Asia. It is possible for Asian players to play with non-Asian players and vice-versa if you have a dynamic IP or a VPN.

In SW2, each server has its own PvP progress. This means that your rank, VOID, PvP set-ups and even your friend list will not transfer to the other server. You will keep all items you have in offline PvE regardless of which server you're on (except a VOID from a given server if you attempt to access the VOID Center while on the other server).

Platform segregation

Android players and iOS players are unable to play with each other. Freyr has stated that this is a technical implementation that they can actually remove, but so far had chosen not to do so due to differing levels of hacking activity between the two operating systems.

Build segregation

As of Version 2.50 on the iOS platform, players running Versions 2.40 or 2.41 are no longer able to play with players running the 2.50 or 2.51 builds of the game. Geographical segregation has been confirmed to affect both groups of players.