16T is used for:
MK4 Swerve Modules with the L4 gear ratio (in conjunction with the 28T-48T double gear).
This L4 ratio also requires a 16T pinion gear on the drive motor.
MK4/4i Swerve Modules with the L3 gear ratio (in conjunction with the 28T-50T double gear).
This L3 ratio also requires a 14T pinion gear on the drive motor. 
17T is used for:
MK4/4i swerve modules for the L2 gear ratio (in conjunction with the 27T-50T double gear).
19T is used for:
MK4/4i swerve modules for the L1 gear ratio (in conjunction with the 25T-50T double gear).