What is partial pressure blending standing for?

Partial pressure blending involves mixing gases by filling a cylinder first with one gas, then another (and sometimes followed by a third gas).

Partial pressure blending is one of three ways in which one can mix nitrox: the others being cotinuous flow blending, and nitrogen filtration.

Partial pressure blending of nitrox involves partially filling a cylinder with oxygen, and then topping it up with air.

One can also use partial pressure blending to blend trimix, by partially filling a cylinder with helium, and then either topping it up with nitrox, or adding pure oxygen and then topping it up with air.