HITI - hi means running, and ti (from tila) means conduit. Hiti actually refers to the conduit made from burnt clay and sometimes wood in this complex water system, but overtime people generally use simply hiti to refer to these spaces as a whole.
GAA HITI - are hiti that are built below road level, so that the spouts can align with natural or manmade underground water conduits leading water from tanks or reservoirs. They comprise of 1 or more spouts built against the wall, set about a meter above ground.
HITIMANGAA - are intricately carved stone spouts often depicting creatures and gods relating to water. They are highly artistic as well as carry religious functions.
DWHON - is the outlet drain that takes the excess water downstream or to places where they can be used for irrigation. The drainage system is a very important and their being able to function is key to the survival of the hiti as a clogged hiti can collapse.
ATHAH - meaning washer bowl, and acts as a distribution bowl. Some conduits have filtration systems engineered into them, where filtered water are then distributed through the bowl. I'm still not entirely clear how the bowl functions, and it seems the athah at the Mangaa Hiti is a recent replacement and is no longer in use.
All of the terms above are from the Newari language, and their functions are mostly based off of descriptions in the UN-HABITAT publication "Water Movement in Patan" 2008. It's a recommended read if you're interested in knowing more about water situations in Patan, as well as the hiti.