The term comes from card processing, where an ISO sells merchant accounts on behalf of a sponsoring bank. Merchant cash advance funders adopted it for the brokers who refer business owners to them, and ISO and broker are often used interchangeably.
An ISO does not fund deals or make credit decisions. It collects your application and documents, sends your file to funders and is usually paid a commission by the funder when a deal funds. Ask any ISO which funders will see your file and how it is paid on your deal.
Openbook Capital is a broker, or ISO. It is not a lender, it names every funder before submitting your file, and it tells you how it is paid on your deal.