D% roll below a skill:
Systems vary, I have seen:
lowest roll is best;
highest roll (whilst still under skill) is best (reflects more skilled people usually do better than less skilled when they actually succeed);
doubles (11, 22, 33) best (when under skill) (this was the old Top Secret iirc.);
take the opposing roll off the skill chance to succeed (usually too complex);
take the opposing skill off the chance to succeed (ditto).

Escalating results (usually D% but can be seen on D20). This was the old Rolemaster system:
Roll d% and add skill. If you roll 96-100 (20) roll again and add. Keep going until you don't roll 96-100. If you roll 01-05 (1) roll again and subract, also keep going if you roll 96-100 subtracting again each time.

Then a completely different system - Lost Souls:
Skill results fall into a defined range from appalling attempt to excellent (I cannot remember the names).
NPC's have a straight rating for an ability, e.g. Good defence, Average attack, Appalling climb).
Player's have a numeric ability - they roll D% and consult a chart based on their ability to get a rating.
The result is determined by the different positions in the ranged of ratings:
eg a Good defence v Average attack = no damage.
Good run v Excellent run means they get away.