1.20 and 1.21 are BPF (Bat Performance Factor) ratings. The higher the number, the more pop the bat will have. Certain leagues and certifications require a certain BPF rating or lower for the bat to be legal for play. Senior Softball allows 1.21 BPF bats, while USSSA only allows 1.20 BPF bats. Because of this, you cannot use a Senior Softball bat for USSSA play.
This version is not approved for USSSA, only ASA, ISF, and NCAA play. The USSSA version is the FP19GHU10.
The 2018 Easton Ghost Double Barrel -10 Fastpitch Softball Bat (FP18GH10) will feature ASA, ISF and NCAA certifications. This model will not have USSSA or NSA certifications.
Most high schools play under ASA rules, making this bat legal for high school play. With that being said, I would recommend confirming this with a coach or league official before purchasing this bat.