Muster is a mysterious substance, occasionally secreted by living beings in a process referred to as passing muster. The rarity of the chemical makes it valuable, though it is seldom used in manufacture and is often replaced with any of a variety of other substances.
Attempts to find sources of muster, e.g. people who pass muster, may be error-prone in the absence of clear and conclusive measurements. It is more common for other substances, or sources of substances other than muster, to pass for muster despite not passing muster in those systematic trials which use careful measurement.
Fringe researchers have on occasion claimed to have identified very powerful sources of muster, though such observations have thus far failed to be repeated; this is sometimes countered with the unverifiable claim that those who pass muster extraordinarily simply have an awful habit of passing by, and do not stay around from one observation to the next.