NCP can handle custom labels that you create on an app in PAS. NCP will create corresponding tags in NSX-T for those labels.

In PAS, you can create labels with the following command. For example,
cf curl v3/apps/<app-guid> -X PATCH -d '{"metadata": {"labels": {"aaa": "111", "bbb": "222"}}
You can also delete a label by setting the value of a label to null, For example,
cf curl v3/apps/<app-guid> -X PATCH -d '{"metadata": {"labels":{"aaa": null}}}'

These commands trigger events that NCP can retrieve. For a new label, for example, "aaa":"111", NCP will create the tag app_label/aaa:111 for the logical ports of all the app instances. If you delete a label, the corresponding tag will be removed.