User details (name, address, etc) are stored across multiple sites that are vendor specific. For example, vendor A holds information concerning room bookings while vendor B holds information specific to cabs, vendor C holds information regarding parking, and the list continues. They also hold credentials specific to each of those services so there isn't a shared credential system or the concept of an identity provider. The information across these systems may hold the same information e.g. name, address, etc
Each of these vendors publish an API or web service of sorts. Since there is now a requirement to aggregate these services however provide users a single journey (as opposed to redirecting them to a vendor specific site), what options are available to authenticate users across all systems.
Bear in mind we have no control over vendor specific web services nor in some instances is there interest from the vendor to change their API to support an identity provider system.