MPLS (Multiprotocol Label Switching)
Updated
MPLS forwards packets along paths set by short labels, instead of a full IP lookup at each hop. Operators use it to build private networks between the sites of a company (MPLS VPNs). Its architecture is described in RFC 3031.
In practice
If your company already has an MPLS network between its sites, traffic from a private APN can join it like one more site, through a link to your network provider.