How to determine the units when reading a multimeter?
No the 'units' around the outside you are referring to are not units but ranges. If you set it to 20m, then the  highest current you can measure its 20mA. If you are on 20mA and it says 0.5, that probably means you are measuring 0.5mA.

There exist auto-ranging multimeters which you simply set to current mode and you don't have to worry about picking a range.