The documentation must be enough to let an experienced service auditor who had no earlier link with the engagement understand what procedures were carried out to comply with ISAE 3402 and with applicable law and regulation, when and to what extent; what those procedures found and what evidence was gathered; and which significant matters came up, what was concluded on them and which significant professional judgements were made in getting there.
The graph holds this control, the 0 it maps to, and the evidence behind each claim, over MCP and REST.