I do not claim that these principles as I have formulated them are accurate, nor do I necessarily want to defend them: rather, they should be seen as a way of trying to make sense of what the Firthians were trying to do.
In general, exponency is a relation between two different levels of linguistic statement.
Material on each side of an exponency statement must be appropriate to its level, and well formed according to a set of principles statable at that level.
Grammatically and phonologically different structures may have identical phonetic exponents. Conversely, identical phonetic stretches may have different phonological analyses, depending on grammatical alternations.
Phonological categories may relate to phonetic exponents in a one-to-many fashion. (If A is a phonological category and A' is the set of phonetic exponents for A, then the various exponents in A are known as co-exponents.)
The statement of phonetic exponency makes (can make) reference to system and structure.
Phonetic exponents can refer to one or more phonetic parameters. If there are several phonetic parameters, there need not be a natural connection between them.
Information about timing is part of phonetic exponency not phonological statement.
Phonetic exponents can be stated compositionally and can be overlaid on one another in time, in the utterance.
The absence of a phonetic feature can count as an exponent.
Phonetic features may serve as the exponent of more than one phonological category at any one time; in doing an analysis, phonetic features are not subtracted from the signal.
You can get more information by e-mailing me: firstname.lastname@example.org