A PPP formula is a method of calculating PPPs. Some other terms used to refer to the same concept are "PPP calculation method", "multilateral index", "aggregation method", "aggregation approach" and "price index" (though "price index" can also refer to bilateral price indices). In the context of validation tables, a PPP formula may be listed under "imputation".
Individual formulas are called X (multilateral) index, X system, X method, or X approach, where X is the name. For example, X can be "Walsh".