Found:

Rule feature

MORPHOLOGY: a type of diacritic feature which triggers (or blocks) the application of a phonological rule. This feature is usually assumed to account for irregular word formation. EXAMPLE: Alternations such as foot:feet, goose:geese and tooth:teeth can be accounted for by assuming that the words foot, goose and tooth, have a rule feature [+U] which triggers the phonological umlaut rule.

LIT.	Chomsky, N. and M. Halle (1968) Kenstowicz, M. and C. Kisseberth (1979) Zonneveld, W. (1978)