SPAD Inhibit short and long

[PJW]

Please explain SPAD inhibit short and long. Working procedure and difference between them.

Normally of the control system detects a SPAD then it alarms and also (to most recent practice) does the equivalent of pulling entrance button of the signals who's routes could be endangered.  However don't want this to happen when the train has been procedurally authorized to pass that signal at danger, so signaller needs to tell the control system in advance not to take such action.
Hence would normally use SIS (that is effective just for ONE movement past the signal).
However when a line is under a possession for many hours and the tracks will be going up and down for all sorts of reasons and totally outside the control of the signaller then they use the SIL to get the control system to ignore those inputs totally for the duration that this is imposed.