Hi Rhill,
The Channel library stores all channel processing settings, including the preamp, gate, PEQ and compressor. There are then individual libraries for the gate, PEQ and compressor which can be accessed by selecting the processing block and pressing the Fn (Library) key.
So it depends on how you want to use the libraries, but as an example:
I may use a channel library for a specific musician, such as their ideal vocal settings, to use as a starting point.
I may use a PEQ library when a musician wants to change between two guitars that require different EQ settings.
I hope this helps!
Thanks
Alex