Can you explain suit purity

In 5 card PLO probably more relevant than in PLO 4 card.

When you are double suited, the question arises whether the 5th card, i.e. the one not part of the double combo of suited cards, is of the same suit as one of the combos.

If the 5th card is of one of the two remaining suits, then it is double suited "pure". Otherwise, you have 3 of the same suit, which reduces slightly the chance to make a flush.

In 4 card PLO obv double suited can only be pure. Single suited could be non pure, like tri-suited or a monotone hand, latter being of course undesirable.