In the auto wheel industry 'offset' is the standard method to spec out a wheel.
In the after market for cars and trucks they often use backset or back spacing.
The whole 5+2 etc thing is a go cart / ATC thing.
Offset is the measurement from the center of the tire to the mounting face of the hub.
If the styling face of the wheel is thicker than the back side it dosent have any impact, the center of the tire is set from the face the tire bead sets up against.
The difficulty with this method is it can be hard to measure the actual wheel to find the tire center which is an imaginary plane in space - you can not hook a tape measure on it.
Back spacing is the distance from the hub mounting face to the inside bead face of the tire.
It is a little easyer to measure and with some simple math you can calculate it in your head (you have to account for the thickness of the inner bead seat area).
The problem is people take short cuts and simply measure from the back edge of the wheel and this gives an inaccurate definition of the part.
The 5+2 nomenclature is a short cut that does not tell the whole.story.
it came about from making wheels from 2 bell shaped halves.
The 1st number is the depth of the outer bell, the 2nd number the depth of the inner bell.
This is an easy to make way to do wheels.
The problem is without the thickness of the material you do not know where the hub mounting face is.
So, a 10mm offset isngoingnto have the hub mounting face 10mm closer to the front as measured from the tire center line.
This will be close to a 3+4 (with out knowing all the details).