I think that the inner seal is the seal that is installed on the housing side, and the outer seal is the seal that is installed in the bearing end of the axle shaft.
I haven't replaced any of the outers myself, I know it takes a press to put them in, but the inners I have replaced multiple times.
I guess since I was going through rear axles with the lockrite, and the spool, since I was just replacing the shafts with other used shafts, I didn't care too much.

Heather