I have some confusion about function SimulcastConsumer::IncreaseLayer()

I’m reading the mediasoup source code , and i have noticed there is a function call in IncreaseLayer() to get bitrate of a stream layer as the following code line.

requiredBitrate = producerRtpStream->GetLayerBitrate(nowMs, 0, temporalLayer);

My confusion is why this function call has a fixed argument (the second argument). and i found in function SvcConsumer::IncreaseLayer(), there also have a similar function call to get layer bitrate, but with a non-fixed spatialLayer. I didn’t quite follow, could you explain it?

What is the motivation of your question? We don’t help here people who takes a look to the mediasoup internals to later build their own privative SFUs. Just in case.