Specification does not contain only items. It can be composed of a series of other services. For example, hair styling consists of a number of separate services:

Setting up compound services in this way allows hairdressers to keep track of how many hair washes have been made. That is, both as separate services and as part of the whole hair styling procedure.

Specifications can contain multiple levels. This can be taken down a level: washing hair requires shampoo, water, and labor. So hair washing could also be set up as a compound service:

This results in a multi-level "specification" for hair styling service: