The drilling BOP Stack as you find configured for your current/next well may be an API BOP stack or a non-API BOP stack.
API Standard 53 'drives' and mandates the minimum requirements for a BOP stack - whether surface or subsea. Providing the minimum mandated requirements are met, the stack is fit-for-purpose and may be considered 'legal for use'. However..............
Many stacks are non-API, inferring that they are configured and constructed 'in excess of the minimum requirements'. The general trend for deep and ultra deepwater is that the stack is supplied with a level of design redundancy that minimizes the need and probability of a stack retrieval due to a component failure that is contstrued as an unacceptable operational risk to the safety critical integrity of the well.
Many UDW stack refinements are actually featured in the architecture of the stack control system.
It is a common prerogative of operators to exercise the BOP stack re-configuration change prior to drilling their well. Most commonly, this will entail the change in position of BOP-mounted valves (choke/kill valves) with respect to BOP ram type preventers' outlets.
Hope this helps, if you need further, please let me know.
Dr P.A.Potter /