An allophonic rule is a phonological rule that says which allophone realizes a phoneme in a given phonemic environment. In other words, an allophonic rule is a rule that converts the phonemes in a phonemic transcription into the allophones of the corresponding phonetic transcription. Every dialect has a set of allophonic rules.

For example, in General American, an ideal accent of American English, the voiceless alveolar stop phoneme is realized as the alveolar flap allophone when it is preceded by a non-lateral sonorant phoneme and, at the same time, followed by an unstressed vowel phoneme:

Phonetics | Phonology