Language pair
A language pair is the combination of a source language and a target language, including the direction between them. English into Arabic and Arabic into English are two different pairs, and a translator's competence, and often their credential, applies to a specific direction.
How it works
A language pair names both languages and the direction between them, written source into target, for example English into Vietnamese. Direction matters: translating into a language is a different skill from translating out of it, and quality, availability and price can differ sharply between the two directions of the same pair.
Machine engines, translator credentials and translation memories are all specific to a direction, which is why work is always scoped per pair and direction rather than per language.
How SourceTarget uses it
SourceTarget scopes and prices every job by language pair and direction, not by language alone, because engine quality, translator availability and credentials all differ between the two directions of a pair. A quote reflects the specific direction you need.