[burntsushi]

> I thought that was an invalid code point.

Surrogate codepoints are indeed valid codepoints. It's just that valid UTF-8 is not allowed to encode surrogate codepoints, so the space of codepoints supported by UTF-8 is actually a subset of all Unicode codepoints. This subset is known as the set of Unicode scalar values. ("All codepoints except for surrogates.")

[Dylan16807]

Those points cannot be validly encoded in any format. I suppose you can argue that they are valid-but-unusable in an abstract sense, since the unicode standard does not actually label any code points as valid/invalid, but if you were going to label any code points as invalid then those would be in the group.

[danbruc]

You are certainly correct that it is common to not pay too much attention to what things are called in the specification, especially if you want to create a fast implementation. Logically you will still go through all the layers even if you operate on only one physical representation.

My admittedly quite limited experience with Unicode is from trying to exploit implementation bugs. And with that focus it is quite natural to pay close attention to the different layers in the specification in order to see where optimized implementations might miss edge cases.

And I am generally a big fan of staying close to the word of standards, if it does not cause unacceptable performance issues, I would always prefer to stick with the terminology of the standard even if it means that there will be transformations that are just the identity.

The distinction between code points and scalar values will for example become relevant if you implement Unicode meta data. There you can query meta data about surrogate code points even if a parser should never produce those code points.