Restricted gameplay or variant gameplay is self-imposed rules or achievement that consists of completing parts of the game while under restrictions of achievement/ruleset.

Examples of restrictions(Nth level reached while..);
1.Not killing monsters
2.Not using weapons
3.Using only Attack Skill X .
4.Using only starting items or normal items of low quality.
5.Not trading
6.Not using chat
7.without leaving specific area (near start zone)
8.Not using Skills/Passives/Auras
9.Without dying
10.without being hit/wounded
11.Without allocating stat points
12.Without allocating skill points
13.Without attacking monsters manually(pets/auras/reflect)