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Assuming Subaru uses valve stem type TPMS sensors they have internal non replaceable batteries. The service life is not known with certainty but is thought to be about 5-7 years, coinciding with expected tire life (nobody should be running tires that are over 7 years old).
They are shipped in dormant or hibernating state to maintain this service life from date of activation. Different TPMS software activates these sensors differently. Some self activate and some require activation codes to be inputted to the TPMS ECU.
Self activating sensors respond to pinging from the TPMS ECU and wake up after a few pings. That's how the sensor and ECU communicate from then on. The codes are different for each sensor in order that the TPMS ECU detects which corner the wheel is on and that combo of sensor code and TPMS code follows that wheel until the code is overwritten by a new sensor install. I know for sure the TPMS ECU can store at least 8 codes and probably more before it will forget a code.
Garage door openers have a similar memory (different coding system) which can fill up if you have programmed too many homelink codes into the same receiver over time. These have an internal code clearing system. I discovered this during a particularly frustrating homelink coding session with a newly acquired car.
If you have trouble with TPMS not activating then perhaps the correct wake up call has not been sent or perhaps your car has had three sets of TPMS equipped wheels installed and the TPMS ECU needs to be cleared. I do not know if Subaru TPMS has a maximum memory capacity for codes but I suspect it might.
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