Ignition Advance Multiplier
The ECU can adjust IAM in response to knock, and this value is used to scale the entire timing advance map. Because it effectively adjusts the entire timing map at once (as opposed to adjusting for a single RPM/load cell), it is presumed to be intended to compensate for varying fuel quality (e.g. if the owner fills up with low-octane fuel).
IAM is just one of several variables that contribute to the total ignition timing.
On 16-bit Subaru ECUs, IAM is an integer that varies between 0 and 16.
On 32-bit Subaru ECUs, IAM is a floating-point value that varies between 0 and 1.