I think NOL may be Nepal govt rule (to at least theoretically control brain drain or to control the unscrupulous workers who chose to work abroad while coming here in govt paid training/education program) . NOL could also be govt to govt agreement which was in paper long ago ( no country can theoretically employ a citizen of another country unless the country of origin agrees to it). Of course,  no one cares about it anymore. But it is a good idea to let Nepal know that you work abroad by using these NOLs. It at least helps others(researchers, policy makers) to get the data.