An offset of 12P means there is 12mm of positive offset.
Here is a (crappy) cross section of a rim, excuse my lack of artistry. See notes below.
Its supposed to be a minilite.....
Take a rim with no tyre on it. Measure the distance where the tyre would normally be, from the front face (BLUE X) to the rear face (YELLOW X). Half of that measurement is the
centre line (GREEN C) of the rim.
12mm offset means that, the face of the rim that presses against your brake hub (ORANGE LINE) is 12mm away from the
centre line of the rim. A positive offset means that the face is 12mm closer to the
rear of the rim (YELLOW X). This makes your wheel sit close to your wheel arch. The higher the number is (in the positive) the wider your wheels will appear.
If it had 12mm negative offset, it would be 24mm the other way ---12mm back to the centre line, then another 12mm towards the shiny side of the rim (BLUE X). This would make the distance from the orange line to the yellow X GREATER than the distance from the organe line to the blue x. The rear edge of the rim will be closer to the suspension and subframe.
Minis with a large amount of positive offset usually need to have wheel arches fitted to cover the wheels.
Large amounts of positive offset will aslo put added strain on wheel bearings, because the wheels are moved AWAY from the bearing.
I hope can you understand what i've said...
Unfortunately i dont know the standard specs of a moke wheel, but someone else here will.....