This returns the value of CHANTYPES from raw 005 when you connect to the irc server.
When you're not connected to a server, mIRC uses a default $chantypes value of #&.
Most IRCd will return # and & where #chan is used for standard channels on that network and &chan is used for local channels. Local channels are only resident to that server and will not share information to other servers on that network. This means you can have multiple &chans with the same name on a network.