One API is a client library that enables you to connect and trade on multiple FICC trading venues. We translate the trading venue APIs into the format used by TransFICC, to allow for easy/single integration – so connecting to a new venue is just a simple configuration change.
We also manage changes to APIs, when trading venues change fields or perform an upgrade. It's a hosted service so hardware and venue connectivity issues are all managed by us.
We use a client library as it provides better documentation and pre-built sample code, making connectivity easier and faster for our clients.
Don't just take our word for it, look at this feedback from one of our clients...