If I'm reading the question right, you mean what position on the guitar neck do you play, depending on what note is mentioned in a piece of actual written music.
For session playing, it's basically experience and common sense. Try several positions, and the one that works and sounds the best is the one you'd use. Occasionally it'll be written in the music itself as a sidebar if they're going for a particular tone.
Most of the time though, the person who wrote it is wanting the correct notes played on a particular type of guitar, and doesn't care how you play it as long as it sounds right.