It's the "RANKING" ie how many chips to make up the memory (1/2 rows, single sided/double sided) - in the servers we use at work they recommend you don't mix ranking in a bank.
I've upgraded both my X2's and my X using bog standard PC memory without any issues (pity I cant say the same for my Naomi net-Dimms) by ensuring they were matched pairs.
For one X2 I had a pair of 2Gb sticks I took out of my PC when I upgraded to a pair of 8Gb ones and they work fine. the other I used the stick I took from the first X2 - worked fine too (but has less ram in than the other).
The X I had to scrounge some old ram from work as I didn't have a pair the right spec, again I swapped the original ram out for a matched pair of 1Gb sticks.