As it says on the tin, you are eligible for promotion after a certain time.
A technician for example, would normally be promoted to L/Cpl after trade training, this is when the clock starts.
A year later they would then be eligible to Cpl and then 4 years later Sgt. As previously stated the starting point is L/Cpl so eligibility to Sgt is actually 5 years from that point and not 4 years from Cpl.
It isn't automatic though, for both promotions they need to be fully qualified and have a strong enough recomendation on their CR's.
Vacancy promotions are for those in the Corps who can earn them where as time promotions are for the Tech Spankers. The same Tech Spankers who would go off and cry if they didn't get promoted at set times even if they are not worth a sh1t!!!!!!!!!!!
Techs only get promoted 'on time' if they are reccomended for promotion on their CR, there are plenty out there who are behind the timeline.
Only weak OCs can get a sh1t Tech promoted 'on time' if they don't deserve it.
The Techs are eligible for promotion on a timescale, regardless of vacancies whereas Artisan trades are eligible for promotion after a set timeframe if there is a vacancy. This is why Tech LSNs are rank ranged from LCpl to Sgt and they do not get substitution pay in these posts.
If the SSgt LSN is vacant, they may get sub pay for that but not generally in fitter section posts.