The OSPF virtual link extends area 0 for a router that is not connected to area 0. As a result, it makes all prefixes in area 0 reachable through an intra-area path; they are not, however, reachable because the path crosses the transit area through which the virtual link is set up to reach the area 0 remote nodes.
The TE database in a router learns all the remote TE links in area 0 from the ABR connected to the transit area, but an intra-area LSP path using these TE links cannot be signaled within area 0 because none of these links are directly connected to this node.
This inter-area LSP feature can identify when the destination of an LSP is reachable using a virtual link. In this case, CSPF automatically computes and signals an inter-area LSP through the ABR nodes that are connected to the transit area.
However, when the ingress LER for the LSP is the ABR connected to the transit area, and the destination of the LSP is the address corresponding to another ABR router ID in that same transit area, CSPF computes and signals an intra-area LSP using the transit area TE links, even when the destination router ID is only part of area 0.