Platforms are independent entities. Users having an openx account can simultaneously invest in multiple platforms. This is a good benefit for new platforms since users don't have to go through the KYC flow again or don't have to store funds on multiple platforms. It is also useful to not worry about the cryptographic security of these platforms since openx handles cryptographic operations. Users on a platform can choose to be any entity as defined by the platform. These roles are not reflected on openx.
Platforms building on openx need to be registered and need to obtain a unique code that enables them to use the platform-platform openx API before being able to import users and other functionality from openx. This API grants access to a privileged set of endpoints and allows platforms to create new openx users, retrieve data related to openx users, get openx constants and more. Platforms would also be vetted by openx admins to ensure their legitimacy in order to maintain the trust users place in Openx.