A “net split” occurs when an individual IRC server is disconnected from the network. Any users on that server will leave their channels. The server shows a message similar to this (Undernet) for each user caught up in the split:
MorningGlory (~MG@xxx.xxx.82.190) Quit (*.net *.split)
To those users, it looks like everyone else left the channels they are in. Sometimes, a user will get Ops in the split channel(s), but it won't last. When that server reconnects to the network, they will lose the Ops. In a channel with Undernet's X, their former status will be restored, including Ops or Voice.