A common definition of URI is that it is a string in format scheme:[//authority]path[?query][#fragment] (from Wikipedia: https://en.wikipedia.org/wiki/Uniform_Resource_Identifier#Definition). So the first part of a URI is called "scheme". But Windows uses the term "protocol" for the same thing (e.g. "Choose default apps by protocol").
What is the exact meaning of these two terms?
The term protocol is used incorrectly in Windows. The first part of a URI is called scheme.