What is the difference between a "voice" and a "register" on an accordion?
For example the Hohner Bravo III says it has 4 "Standard bass voices" and 3 "Standard bass registers"
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Voice refers to how many different sets of reeds the accordion contains. Each set of reeds produces a different timbre, similar to the way sets of pipes produce different timbres in a pipe organ. So "Four standard bass voices" means there are four different bass timbres available.
Register is combinations of reeds that are producing tones at the same time. Again analogous to a pipe organ in which combinations of different pipes are opened to produce a particular sound, accordions will have preset combinations of reeds that are selected by the push of a button. Thus, "Three standard bass registers" means that there are three different combinations of the bass voices available.
See, for example, http://www.accordionlounge.com/accordiontypes.html.
A "voice" is a reed set. When engaging the "master" registration, every reed set is active, and the number of reed sets will indicate how many reeds sound for every key or bass button pressed. An accordion with LMMH tuning has 4 reed sets or voices: "low" (sometimes written as 16" in analogy to organ pipes), two "M" voices ("medium" or 8", usually the main reed set that is on-pitch and the tremolo reed set that is tuned more or less sharp in order to create a beating sound quality) and an "H" voice ("high" or "piccolo" or 4") that will make the sound more piercing in connection with other reed sets.
When there are 3 M reed sets, they are usually in "musette tunning" with one reed set tuned straight, one somewhat flat, and one quite sharp. That generates the piercing sound quality known from Parisian Musette music. Less conspicuous 3-reed tunings are also available, like the still comparatively prominent "Oberkrainer" tuning.
The registers are combinations of reed sets that can be engaged at one time, usually with a combination register switch. On an LMMH accordion, 11 registers are customary. From the possible 2⁴=16 combinations, the silent registration is omitted, and all combinations using the tremolo reed set but not the main reed set.
For a standard Stradella bass, bass voices are split into actual bass reed sets (typically two that are an octave apart) with a size of 12 notes each, and chord reed sets also with a size of 12 notes each. The mechanics assemble chords from 3 notes in each active chord reed set (with the chord inversion predetermined by the chord reed set), and play bass notes by sounding all active bass reed sets and usually all active chord reed sets (very few accordions, like the Hohner Gola, don't do this coupling of basses into the chord reeds, instead having an additional bass reed set).
Just where reed set start on the bass is a black art. Typical large Hohner accordions have bass reed sets starting at E1 and E2, and chord reed sets on E3, E4, E5. Italian models can have something like A1, A2 on bass and F♯3, C4, C5 on chord reeds. Having 3 different octave wraparounds better masks the octave breaks in the various reed sets in the master registration and makes a bass note scale wrap around less conspicuously since there are, after all, only 12 different bass buttons to play.
Just which bass and chord reed sets (= voices) the available bass registers engage again is "voodoo" that everyone does differently. Apart from a "Master" register, it is common to have one with the lowest bass and chord reeds switched off, and one with the highest one or two chord reeds off. The more registers there are, the less common ground with other manufacturers or even models will be.