Not a game mode, this is the entry point. Velocity sends every player here after authentication, in adventure mode with PvP off and building blocked. Your hotbar carries three items: an info book, a nether star that opens the server selector, and a toggle to hide other players. Walking into a nether portal opens the same selector, which is a plugin written for the network. Staff tools and anticheat run here too.
The other modes
SkyBlock
Solo, duo, trio or squad islands, each with its own nether and end.
in developmentBedWars
Four maps are built, but the arenas are not set up, so no match can start yet.
playablePrison
Mine, rank up from A to Z, claim your own cell. Three currencies and one boss.
playableCreative
Claim a 64x64 plot, build with unlimited blocks, keep it protected.
playableSurvival
Claim land, build an economy, trade with other players. PvP is on.
playablePvP
1.8-style combat: no attack cooldown, sword blocking, old golden apples.