Taz Pro Nozzle offsets

What do the nozzle offsets actually represent, specifically the Z one.

I have dialed in extruder 1 using a Zoffset of -1.35. This gives me an excellent 1st layer and good adhesion.

When I print with extruder 2, it is clear I need to adjust to get the same good results. Do I adjust the “nozzle offset Z” to do this? The factory number for this is -1.56. The name makes it sound like this means extruder 2 is -1.56mm Offset to extruder 1, but that sounds insane. Is it an absolute number rather than a relative one?