Squad Battle

Squad Battle is a mode in Splatoon made available as part of the Version 2.0.0 DLC. Squad Battle allows groups of up to four friends to participate together in Ranked Battle with other squads. A squad will always battle together on the same team, and players who are rank S or S+ will not gain or lose rank points.

Matchups
When forming a squad, players can create a Quad Squad (four members), a Tri Squad (three members), or a Twin Squad (two members). Squad Battles are always either 4v4, 4v3, or 3v3 battles, since Twin Squads cannot fight alone - they are paired up with another Twin Squad for a four-player team. A Tri Squad, however, is treated as a Quad Squad, to their disadvantage when facing actual Quad Squads. However, in the event that they would be outnumbered, the Tri Squad is granted a quicker respawn.

Passwords
If necessary, a squad can be protected by a four-digit password, allowing the creator of the squad to limit it to just invited friends. However, the friends-only rule still stands, so even if someone knows the password to a squad, they cannot join unless they are friends with the person who created it.

Trivia

 * Nintendo has set it up so you can play one of the Squid Sisters' songs, Calamari Inkantation, using the sounds produced from the individual digits when you set up a password.