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 via an intra-area path but in reality, they are not 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 of 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 is directly connected to this node.
This inter-area LSP feature can identify when the destination of an LSP is reachable via a virtual link. In that case, CSPF automatically computes and signals an inter-area LSP via the ABR nodes that is 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.