"A" language: The interpreter's native language (or another language strictly equivalent to a native language), into which the interpreter works from all her or his other languages in both modes of interpretation, simultaneous and consecutive. All interpreters must have at least one 'A' language but may have more than one.
"B" language: A language other than the interpreter's native language, of which she or he has a perfect command and into which she or he works from one or more of her or his other languages. Some interpreters work into a 'B' language in only one of the two modes of interpretation.
"C" language: Languages, of which the interpreter has a complete understanding and from which she or he works.
If an interpreter has a near-native command of his or her "B" language and offers other less common languages, European institutions occasionally certify an interpreter's "B" language as "B-retour" upon which his or her interpretation into his or her B-retour language may be used as a relay for further interpretation into other languages. To give you an example: As there weren't enough native speakers of EU languages who could interpret from Czech during negotiations of the Czech Republic's accession to the European Union, Czech interpreters with a "B-retour" interpreted from Czech into English and French, through which the Czech speeches were interpreted further into the rest of the working languages. In other words, a Finnish interpreter interpreted the original Czech speech from either the English or the French rendition.