Pop Tag is a MPLS label which asks the penultimate hop to remove the label and send it to the edge router as a native IP packet. Hence Edge will have to do only IP lookup and no label disposition. Untagged is when there is no outgoing label at all. You would get Untagged lable for routes to NON MPLS interfaces or when LDP is down with ur neighbor.
The aggregate label indicates that the arriving MPLS packet must be switched through an IP lookup to find the next hop and the outgoing interface. You would generally find this in MPLS TE tunnels. Iam assuming recursive is more to do with CEF.
