I believe they are when you attack/stall behind Substitute.
Users of the Sub stall sort can be SubSeed (Substitute+ Leech Seed) users like Serperior, or Boosters like Swords Dance Speed Boost Ninjask. Sometimes they are Toxic Stallers too. Usually these Pokémon carry Leftovers for recovering the Substitute damage.
The most notable sub attacker is Breloom with its famous STAB SubPunch (Focus Punch+ Substitute, sometimes with Leech Seed and Spore, in Breloom’s case) set. Ninjask fits this category well too, as it usually attacks behind Substitute after getting some boosts to its speed and attack (via Speed Boost and Swords Dance).
So, really, there are different types of “Sub Attackers” and “Sub Stallers”, so they are broken up into smaller sections of SubBoosters, SubSeeders, and SubPunchers, as well as more
Hope this helps! :)