Custom techniques are those created solely by the ninja without a specific grouping, and thus are used typically solely by the ninja who created it and those he teaches it to. Custom are often seen funner as they give your abilities a certain sense of uniqueness and allow you to further develop your character.
Ninjas may make custom jutsus, however, are limited by their rank. A Genin may make up to D-rank Custom Jutsus, a Chunnin C-rank, an ANBU Chunnin B-rank, and all ranks above may make any rank customs. Customs cost no more additional JP nor count for specific slots, but DO typically require more RP effort to learn.