Graham Simons has it right: the College of Arms clears the badges, devices, colours and motto’s.
However, the rules can also be bent making unofficial/official badges and or motto’s.
Photo shows a Hawker Demon of 607 Squadron: the device was granted the motto was not. The squadron want the word ‘UP’ the herald would not allow it. After some tooing and froing between squadron CO Runciman and the Herald they decided there should be no official motto.
The device was retained but was depicted in the Fighter Command arrowhead turned to the UP position rather than straight ahead: Unofficially 607 Squadron had it’s motto ‘UP’.