When you retrieve a var or property, "THISACTOR" is a magic reference to the sprite ID of the actor running that code. But if you have another sprite's ID, you can use that instead to get or set data on it instead. A few examples:
getav[PARENT].TEMP2 TEMP // where TEMP and TEMP2 are per-actor vars, retrieve PARENT's current value of TEMP2 to the sprite making the call's value of TEMP geta[PARENT].extra TEMP // Retrieve PARENT's extra value, IE their current HP seta[PARENT].xrepeat 0 // Force xrepeat to 0... seta[PARENT].statnum STAT_ACTOR // ... and statnum to STAT_ACTOR, essentially doing a remote killit on PARENT
This is also why you need to do validation on the value of PARENT because if the sprite referenced in PARENT goes away (via killit or other methods), you'll get fucky behavior and/or errors. Also, I'd suggest defaulting the value of PARENT to -1 because technically 0 could be a valid sprite ID, meaning you can check for the -1 value to bail out early on if the sprite is somehow spawned invalidly.