In phonology, a register or pitch register is a prosodic feature of syllables in certain languages, in which tone, vowel phonation, glottalization, or similar features depend upon each other. Burmese, Vietnamese and Wu Chinese have such systems.
Register (phonology) Wikipedia
In Burmese, differences in tone correlate with vowel phonation so neither exists independently. There are three registers in Burmese, which have traditionally been considered three of the four 'tones'. (The fourth is not a tone at all, but a closed syllable, called "entering tone" in translations of Chinese phonetics). Jones (1986) views the differences asresulting from the intersection of both pitch registers and voice registers […] Clearly Burmese is not tonal in the same sense as such other languages and therefore requires a different concept, namely that of pitch register.
Similarly, several Vietnamese 'tones' are largely distinguished by characteristics other than pitch. For example, in Northern Vietnamese, the ngã 'tone' is distinguished from the sắc primarily by the presence of a glottalization in the vowel, while the nặng and huyền syllables are distinguished primarily by having a short creaky vowel versus a long breathy vowel.
Khmer is sometimes considered to be a register language. It has also been called a "restructured register language" because both its pitch and phonation can be considered allophonic: If they are ignored, the phonemic distinctions they carry remain as a difference in diphthongs and vowel length.
An example of a non-Asian language with register distinctions is Latvian, at least in the central dialects underlying the standard form. Long vowels in stressed syllables are often said to take one of three pitch accents, conventionally called rising, falling, and broken. However, the "broken tone" is distinguished by not pitch but the presence of glottalization, much like the ngã register of Northern Vietnamese.