This server is a Service Metadata Publisher (SMP). For each of the participants registered on it, it states which document types that participant is able to receive, and at which Access Point these documents are to be delivered. We call those the 'receiving capabilities'.
That information is served via the Peppol SMP REST API - the interface a sending Access Point queries before it delivers a document. A query needs nothing but the identifier of the participant in question and returns a machine readable XML document.
This SMP is powered by phoss SMP, an Open Source solution. Everybody is free to use it, to review what it does and to run an own instance of it.