What does linguistic classification mean for interpreters?

The A language is the interpreter’s native language, in which they have the highest level of proficiency, typically the interpreter’s mother tongue. The B language is one in which the interpreter is fluent but isn’t a native speaker. Interpreters have a high level of comprehension and expression in this language.
Interpreters generally achieve optimal performance when working from their B language into their A language (i.e., from a non-native language into their native language).
In addition, there’s a C language classification. This is a language that the interpreter understands well enough to work from into their A or B language. However, they lack the skills to interpret into this C language, also known as a passive language.