"Activateifused" is a command in the script to activate the name of the entity specified in the IFUsed field of an entity or trigger.
"Appearifactivate.fpi" is the start script you would assign things like a trigger, story zone, win zone, sound zone, decals, etc. The script waits for the element to be activated and then it "activates" or appears.
For example, I script my battery so that once I pick it up, it has the "activateifused=1" command in the script. In the properties for the battery, I set the name of the trigger to be activated. I place the trigger where I need it and edit the properties for it by assigning the "appearifactivate.fpi" to the Start script for the trigger.
In case you need the script for "appearifactivate.fpi, here it is:
;Artificial Intelligence Script
;Header
desc = Instant On
;Triggers
:state=0,activated=1:spawnon,setalphafade=100,runfpidefault=1
;End of Script
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