what does the +10 mean in math terms?

+10 = + 10 digits. What a digit means depends on the measurement range. In the tables you supplied, it is stated for example: range = 220 µA, resolution = 0.01µA. This means 1 digit = 0.01 µA or 10 digits = 0.1 µA. This is an absolute error that has to be taken into account regardless of the reading.

Therefore with a small reading the relative error is higher than with a high reading as, to stay within this example, 10 digits = 0.1 µA is a high relative error of 10 % when measuring 1 µA (within the 220 µA range) but is very small when measuring 100 µA (still within the 220 µA range) where it contributes only 0.1 %.

Im not sure what you mean by "absolute voltage"

the steps are discrete and range from 0.0015V to 0.0465V

It is easy to measure a voltage step of 1.5 mV when the instrument is set to the 220 mV range.

If your absolute voltage is e.g. 10 V and you want to measure the same step (from 10 V to 10.0015 V), you need to set the meter to the 22 V range. In the 22 V range the resolution is 1 mV (=0.001 V). Taking into account an absolute error of 2 digits = 2 mV and a relative error of 0.1 % => 10 mV, the total error when measuring 10 V in the 22 V range is 12 mV which renders the measurement of a voltage step of 1.5 mV useless.

If, however, your max. voltage is 0.0465 V, as I assume from post #7, you can set the meter to the 220 mV range with a max. error of 0.1% of 46.5 mV = 0.0465 mV plus 5 digits = 0.05 mV. In total 0.1 mV (generously rounded up). Your smallest value to be measured is 1.5 mV, therefore the max. error is on the order of 6.5 %.