What is reasonably practicable may depend on the level of participation of others, and a disagreement may arise on how much consultation, cooperation and coordination is required. A PCBU should not simply accept inadequate action by another duty holder. It should check they know of the duty and what the PCBU considers necessary to comply. Written arrangements are not essential but help clarify expectations, and contracts should include a requirement for other parties to consult, cooperate and coordinate on safety, giving the PCBU a contractual right to enforce it.
The graph holds this control, the 0 it maps to, and the evidence behind each claim, over MCP and REST.