On paper a Class B+
ONT can detect till -28 dBm and a Class C+ ONT (more expensive) can detect till -32 dBm. Most ONTs are Class B+.
However in reality even Class B+ ONTs seem to work okay with input level less than -28 dBm.
One thing, I'm still not very clear about is whether max throughput or performance affected by the input signal levels. If the ONT can detect the signal is it all the same?
The more passive splitters the LCO/ISP add the power level drops, also as the distance increases the power drops. I guess it is good to keep an eye on the power levels cause if it's on the extreme end, if another splitter is added you might lose connectivity.