What is unit_type_bits used for? How should I set it if I want to create a new map with 3 different types of units?

[lonelyhat]

I want to create a customized map and add in 3 different types of units, however I don't know how to set the unit_type_bits. Can someone help me with this? Also may I know how I should create a new customized unit based on existing unit and add it to the map? Any instruction or tutorial? Thanks