They had different models with different capabilities. As they made minor style changes, they bumped the numbers slightly. The 81–82–83–84 were basically the same concept, as were the 85–86. The 89 and 92 were higher-end models. The 80 and 73 are simpler models intended for middle school.

All of them are basically a multi-generational scam perpetrated against the hapless parents of American high school students who were told that they needed to buy overpriced anachronistic calculators for their kids to succeed in school. In my opinion the calculators have overall caused more pedagogical harm than benefit; the students would be better served by some combination of (a) problems that can be solved without the tedious but trivial numerical calculations these calculators support, or (b) are solved using a real programming language. If someone really wants to assign simple numerical problems, give the kids slide rules.

Calculators of this type used to make sense for an engineer doing work in the field somewhere, but make no sense in the context of a classroom.