Subject to it or affected it - African Customary Law
Mostly one of the parties must be subject to it or affected through it. Whether as the plaintiff and the defendant relating to the same ethnic group that they may be said to be "subject" to the customs of that ethnic group that could then be applied to settle the dispute. As like as situation, a dispute between Kikuyus relating to any of the matters listed earlier cannot be settled under Kamba, Luo or any other customary law except Kikuyu customary law.
Therefore, whether there is a dispute involving parties from different ethnic groups it may be determined according to the customs of either party, since the other party would be "affected" through the custom.