User ID vs Visitor ID

BACKGROUND: Matomo clearly distinguishes between users and visitors on the one hand and uses these same terms interchangeably on the other. The result is, of course, confusion. In particular, Matomo uses the word USER in conjunction with those who make use of the reporting API, and uses the word VISITOR in conjunction with those whose behavior is observed and recorded when they visit a website. It then performs a complete about-face and devotes an entire section in the manual to something called User ID that appears to deal with the handling of visitors in the context just described. Within this section alone it then speaks of cid and uid.

Now, I am fairly confident that coders like mathematicians enjoy the precision of coding – even when a single missing comma or a colon pretending to be a semi-colon can take hours of one’s time to discover and repair. Also, I am fully sympathetic with those who insist that the rules of English grammar taught by grammarians in school often do not square well with the reality of written, and especially, spoken English. This said, we are dealing here with two specific words with very distinct meanings in the English-speak of Matomo.

QUESTION ONE: In light of the above elaboration how are we the Matomo User Manual VISITORS to interpret the word User in the section entitled User ID?
QUESTION TWO: Can the variables cid and uid be used with both VISITORS and USERS, or are these variables reserved for VISITORS only?

DISCLAIMER: If you have difficulty responding, it is well understandable, and I will be very patient until I fully understand.