I think rads was convenient for a game term as it was easy/convenient to identify as a dose of radiation. The term
rad is an acronym for radiation absorbed dose and is a measure of the energy deposited onto/into living tissue.
The term mrem is an older measurement of radiation, it is one thousandth of a rem with mrem meaning
milli Roentgen equivalent in man. I think mrem was mostly used within the USA.
Both are valid terms but with several different terms used to describe a does of radiation it all becomes quite unclear and seems just a little screwy.
There's a page describing all this in far better terms than my efforts on the following link: -
http://ieer.org/resource/classroom/m...n-terminology/