Here are the mission ranks from F=Lowest SS King Only=Highest
F-class
E-class
D-class
C-class
B-class
A-class
S-class
X-class
X-B-class
X-A-class
X-S-class
Commanders Only-class
Royal Gurad Only-class
Soul Socitey King Only-class
F-class
E-class
D-class
C-class
B-class
A-class
S-class
X-class
X-B-class
X-A-class
X-S-class
Commanders Only-class
Royal Gurad Only-class
Soul Socitey King Only-class