From Brave Nine Wiki
The restrictions are as follows:
- Mercenaries with original 5 star rank are prohibited from being used.
- Only 3 mercenaries with original 4 star rank can be used.
- The rest of the formation must consist of mercenaries with original 3 star rank.
- Each week, there are four mercenaries selected to be restricted. These mercenaries cannot be used and will not participate if entering a match with them.