Allomorphy rule

Allomorphy rule is a special type of word formation rule (adjustment rule) proposed by Aronoff (1976) to account for allomorphic variation. Allomorphy rules apply to the output of the word formation rules, and instantiate phonological changes in certain morphemes in the immediate environment of some other morphemes.

Example
Aronoff assumes for English an allomorphy rule which changes the verbal suffix -fy (amplify, electrify) into -fic- if it is followed by the noun-forming suffix -ation (amplification, electrification).

Link
Utrecht Lexicon of Linguistics