"Free Battles" are ones that you can set rules to and challenge people with. By default, they have a Level 50 Cap and allow all Pokémon, but it can easily be set to no Special Pokémon allowed (or the players could choose not to use Special Pokémon). :P
That would be "USUM Singles, Special Pokémon Banned, Level 50 Cap" as a format, but most people say USUM Singles in a title and describe the rest with the team. There is no format for this on Smogon; Smogon only has Video Game Championships [Year], Battle Spot Singles, Battle Spot Doubles, and Anything Goes for USUM DS formats. Like I said, you can basically play this on Showdown! by choosing AG and making sure nobody uses Special Pokémon as well as set their Pokémon to level 50.
If you want to know this for an RMT post, you could just say what rules you are using in the post itself. :P
Hope I helped!