Does anyone know what exactly is imperium in TWW scripts?

I could only find one thread, where it is implied to be the number of fully controlled provinces, which is not true according to my tests (tested using chaos invasion script). Neither it is the chapter number, which also was was hinted at in search results (tested the same way). From what I gathered, it works like imperium from R2/ATW or fame from S2, but are there actual values available somewhere? Or at least everything that affects it.