(from http://www.ogf.org/pipermail/nml-wg/2012-May/000954.html)

NML currently defines "resource labels", eg.:
* Ethernet VLAN
* Ethernet I-SID
* Frequency on DWDM / Wavelength on CWDM
* ATM VPC
* ATM VPI
* SONET/SDH STS3c/STM/AUG-1 timeslot
* MPLS shim label
perhaps even:
* SSID on a wifi
* strand in a fiber bundle
* ...

This label is used for both:
* distinguishing between flows on a link (aka channels)
* routing and switching (eg. "switch X will forward data from port 1, label 28 to port 4, label 42")

The "destination labels", such as destination IP address or destination MAC address are also used for routing and switching, just like the resource 
labels above.

I previously presumed that destination labels could be described the same way as resource labels.
I'm longer sure that this is a good idea.

Recall that each Port is associated with exactly one label.

For a host with one IP address 2001:0DB8:B4C6:6AAE::1 this means that is has one ingress Port (for this IP address). On the other hand, it would have 
2^128-1 egress Ports (for all possible IP addresses that is can send to).

This discrepancy between ingress Port and egress Ports seems odd to me, and makes me doubt that destination labels are the same things as resource 
labels.

G.800 thinks a resource label is a different beast than a destination label: it associates resource labels with the adaptation, while is associates 
source- and destination labels with the termination.

That's all nice, but I'm still at loss how to describe source- and destination labels in NML and how to deal with them.