Delineating between security property requirements and internal consistency requirements in detailed client workflow

There are parts of the specification which implementers SHOULD adhere to in order to gain the security properties of TUF.
There are parts of the specification which implementers need not strictly adhere to, so long as they are internally consistent across the detailed client workflow.

For example, it is not strictly necessary to persist metadata files in the recommended fixed form. However, it is necessary to store the data in a consistent and consistently identifiable fashion in order to be able to implement the workflow.

It could be helpful if the specification explicitly delineated between these two types of requirement so that implementers can more easily make informed decisions. In many cases the delineation is implicit (through use of RFC 2119 keywords on pieces of the detailed client workflow related to TUF security properties), but more explicit notation/annotations would make it easier for integrators to adopt TUF safely.

This may be easier to do after #121

Review the specification's detailed client workflow and its use of RFC 2119 keywords, then compare the context from issue #121. Define how security-property requirements should be distinguished from internal-consistency requirements, with annotations that let implementers identify which choices are flexible; done when the distinction is explicit throughout the relevant workflow.