A Customary sets forth a description of the custom of a particular place, in a particular time. It is not an exhaustive guide to the liturgy, because it does not purport to replace the rubrics and directions of the rite being used. Instead, it indicates which options a particular community uses among those available in the prayer book, provides direction for adapting the liturgy to a specific liturgical space, and fills in some of the silences in the prayer book's rubrics. This Customary should therefore be read alongside the prayer book. The answer to many questions about worship can often be found by starting first with the prayer book and then turning to the Customary.
You can see the entire customary for the Chapel of the Apostles here.
