Basically, if a kid coming out of high school didn’t have the necessary credits to qualify for eligibility, they could take a redshirt year to get everything in order. The Big 8 was the only major conference to allow partial qualifiers, though I’m not sure every school did. Partial qualifiers had a choice between JUCO and a very few major schools that could or would take them in an era where JUCO transfers weren’t really a thing.
When the Big 12 formed, Nebraska was the only school that wanted to keep the old Big 8 rules - even Texas and Oklahoma wanted to up the requirements. If I’m not mistaken, Nebraska’s final recruiting class with partial qualifiers got smoked by Miami in their final games as seniors in the 2001 National Championship Game, which I’m sure is just an AMAZING coincidence that they haven’t been anywhere close to back since.