An allophonic rule is a phonological rule that indicates 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 language has a set of allophonic rules.
In American English, the voiceless alveolar stop phoneme /t/ is realized as the alveolar flap allophone [ɾ] when it is preceded by a sonorant phoneme other than an alveolar nasal or lateral, and, at the same time, followed by an unstressed vowel phoneme.
/t/ → [ɾ] | /+son -lat/ ＿ /+vwl -str/