This information is available in the manual on page EN-76 in a somewhat abbreviated form. Also take a look at the Appendix, page 14. I like the idea of this table quite a bit. I tells you which settings are stored in registrations, which are restored after restart, when you press the "Grand Piano" button etc. It helps to figure out how to set things up. As far as Line in volume is concerned, it isn't tied to a registration. It's sort of global and it can be restored after restart if you enable the "Auto resume" option.
Note that the Line in in PX-560 accepts a pretty low signal. Manual specifies 200 mV (page EN-92), which is -11.7 dBu or -14 dBV. I found that it can take a bit more maybe 300-400 mV. If the signal is hotter, it clips, which can be heard as distortion of the high-volume content. Adjusting Line in volume does not help with that, since the clipping seems to appear before the volume is applied. You need to adjust the volume of the source appropriately.
You may also want to take a look at the diagram on page A-6. It shows how the input from Line in flows, also in comparison with the simpler Audio in. I can add that Line in has an adjustable noise gate (3 levels), while Audio in doesn't. Also, Line in is recorded by the internal audio recorder, while Audio in isn't.