When you create the room, you have:
Available Slots (top is public, bottom is private, not sure what the final option is)
<unavailable option 1>
<unavailable option 2>
Conditions (so many here, dear god....)
My guess is that unavailable option 1 removes the weapon level limit, the first two symbols: 武器 - refer to weapon. The second unavailable option is armor (アーマー).
As for how to unlock, my guess - finish the story in online mode.