The value of the endpoint URI used by an SCA service or reference binding is defined in the varrious SCA Specifications. The Assembly specification, page 53, section 1.7.2 "Form of the URI of a Deployed Binding" defines the overall rules, and specific binding specifications may modify or add addtional rules, for example, the Web service binding specifiction on page 3, section 2.1.1 "Endpoint URI resolution" adds further web service related endpoint rules.
The Tuscany runtime and binding implementations aim to all work consistently so that once you understand the basic endpoint URI rules and conventions it should be easy to determine an actual endpoint from reading the composite SCDL.