Anyway, on to register. This is a term that is associated with the phenomenon of having several styles of speech depending on the situation that the speaker is in. Most of the time, the speaker is faced with varying environments in speech, and the environment affects what type of speech the speaker engages in.
Let me illustrate this by example. A parent for example will talk to a toddler in what is called motherese, this high-pitched sing-song voice intended to be used targeting babies. This can be a type of register. The same speaker will talk differently to an older child. And the same speaker will also talk differently to a person of authority, for example, the President of the country.
Now why is this interesting to sociolinguists? Because several linguistic variables come into play whenever people move from one register to another. For example, I noticed that my father uses certain vocabulary in Tagalog whenever he prays. It seems that certain lexical entries that have commonplace meanings are only used in this environment, that is, speaking to an entity of perceived authority. One example is the lexical item suliranin, which is the Tagalog word for problem. I never hear my father use this word aside from praying. Whenever he needs to talk about a problem in other environments, then the Spanish loanword problema is used.
Linguistic politeness plays a huge role in shifting between registers. Yep, depending on one’s culture, certain registers dictate that speakers use politeness devices. Most languages have grammatical ways of indicating politeness, and these are used depending on what register the person is speaking in.
Due to the fact that I am not specializing in this area of research, I do not know to what extent certain linguistic variables change depending on what register the person is in. Perhaps, there is a language that dictates that the intonation contour should change depending on what register a person is in. That is quite imaginable. A more radical and hard-to-imagine scenario would be a language that dictates that certain sounds should not be used in a certain register. That would be hard, as the language would become lipogrammatic in nature. Perhaps these languages do not exist, but what do I know, as I am not a typologist.