Actually all the surnames of chhetris(khas) are derived from posts they possessed during Jumli Malla (means Strong for duel ) kingdom.
Basnyat--- settler of new annexed/ explored places
bista --- tax collector
Karki - external affairs officers
Thapa -- Internal affairs officer (defense and buruecracy)
Khadka --- Planner for attacking other states
Mahat --- Chief of Army
Kathayat (Kadayat) --- transporter
Katwal -- Information/ Messenger

Originally, all the Chhetris hail from Karnali region (Baise rajyas). Later they migrated to different areas and worked as state army. The chief of army of Bara (Simraungadh) was Swaroop Singh Karki, who went to Kathmandu with a huge army to support Jaya Prakash Mall, but on the way to Kathmandu he decided to back Gorkhali King P N Shah. He was the leader during Gorkhali unification for the western forts like Tanahu, Argha, Khanchi, etc. Even the Kirtipure army was comprised of Khas, Magars and Yadavs of Terai while army of Kathmandu malla kings were Nagarkoti.

Basnet and Basnyat are same. some of them migrated to Gorkha (Sirupali Basnyat: Shivaram singh, Kehar singh, Abhiman singh, Dhaukal singh, Nahar Singh etc who lived in Asan after annexation of ktm) and some settled in Khaptad (Doti and some other districts) region and called as Khaptadi basnyat (EX CoAS Singha Bahadur Basnyat). Most of the chhetris are known by the place they hail from. eg. Kalikote khadaka, Kalikote Mahat, Palpali Khadka etc.

So, Happy 2009 be clear on this.