Time in grade
As a general rule, a person at a particular rank or rate must remain at that rank or rate for a certain minimum period of time even if they meet all other qualifications for further advancement. The concept is that this will ensure the person has time to gain the necessary real-world experience to prepare them for the next rank or rate.
Every military organization employs this concept, if not necessarily by this name, as part of the criteria for promotions, and TRMN uses it as well as it is modelled on a military structure.
The time in grade requirements for each rank/rate within TRMN and its component branches of service are given in Admiralty Orders. The most current order on the subject of TIG is AO 1707-01, issued 01 August 2017.