As some of you may know, phone manufacturers like to give their phones codenames whilst they're developing them, to try to keep it confidential what the final product name will be. HMD and FIH are similar, they have given their models different codenames. These codenames often then get used in the developer community to make it clear which model they are working on (since some manufacturers use the same model name for many different actual models, e.g. Moto G.). HMD, for now, keeps it's model names clear (ish) so, each model name refers to a different model (they also have model numbers for each variant of a model, but these are generally very close so developers can ignore the differences).
Here are the codenames that I think HMD currently uses - the ones that I know at least. I don't know any for the feature phones. I've noticed a bit of a pattern, the codenames follow an alphabetical order, so the flagship is A, with the next top model being B, C, D, etc. I'm not sure this patter is exact, there appear to be some oddities, and the newer models use a totally different approach which I've not worked out the pattern for yet.