Originally we removed offsets and Match changed the property mappings. Later we had to bring back offsets to keep all the functionality of the old transform constraint. Match now only sets the offsets.
The property mappings are up to you. They describe unit conversion from one property to another. If you aren't using clamp, then the exact ranges don't matter because the values are extrapolated. For example, you can map 0-100 to 0-100 and get a mapping with 100% scale (ie 1:1). Values outside the range being extrapolated (not clamped) means if the source value is out of range, say 500, the target value is 500. If you map 0-100 to 0-50 you get 50%: the target bone moves half as much as the source bone, 500 source gives 250 target. If you map 34-72 (a width of 38) to 143-162 (a width of 19) then you also get 50% scale (19/39=0.5) and it will behave identically. Those numbers are harder to read, so it's convenient to use easier to read ranges like 0-100.
If you are using clamp then the target values don't go out of the specified range, so you need to be much more careful what ranges you set. It depends on the range of motions you want to map, so we can't set them for you. We may add some visualization of the ranges though. Enable rulers (ctrl+shift+R) and take note of the values for the ranges you want. Make sure to look at the correct axes based on if you have Local checked. Make your ranges work with a simple constraint before making it more complex.
If you enable clamp and nothing moves, likely your target (right side) values are out of the clamp range. Look at your ranges more closely.
If your mapping causes the bone to move in setup, it's because the source value when put through your mapping does not equal the unconstrained target value. Set your ranges more carefully!
For example, say the source is 100 in setup and the target is 200. I want the target to move +/-30 when the source moves +/-15. I would set the ranges: 85-115 to 170-230. This mapping causes 100 to be mapped to 200, so the setup pose doesn't change, and the mapping also describes the amount of movement I want clamped.
Your ranges don't have to center the setup values. For example, same setup values 100 source, 200 target. This time I map 90-120 (which is -10 and +20 of the 100 setup) to 195-210 (-5 and +10 of the 200 target). With this mapping 100 source is still 200 target, but the range is asymmetrical relative to the setup values.
