Here's an overview of the standard:
This International Standard specifies the requirements for subsurface sucker rod pumps and fittings used in the petroleum, petrochemical, and natural gas industries. iso 23251 pdf
ISO 23251 is a standard related to the petroleum, petrochemical, and natural gas industries - Drilling and production equipment - Subsurface sucker rod pumps and fittings. Here's an overview of the standard: This International