Trains are given a regular loco link for their run. If sometimes happen that regular loco is not avbl prompting them to use the available loco. It is termed as loco offlink.
For eg: Mumbai Rajdhani is hauled by WAP 5 or WAP 7 of Ghaziabad shed regularly. If the loco is not available may be any reason maintenance or loco failure etc and if its given say WAM 1 or say WDM 3D which was available for a particular run, we term it loco offlink.