From ErfWiki

[edit] Proposed Canon

Loyalty is one of applications of Natural Thinkamancy. All units are subject to Natural Thinkamancies: Obedience, Loyalty, Duty, and others.^Erf-b1-p084a

Loyalty is an unknowable unit stat, affecting how likely a unit is to defect or double-deal when possible. Thinkamancy spells can modify Loyalty.