Hi, so unfortunately we can't provide official user support specifically for codes listed on the "ITensor codes" page. This is because those codes are written by a variety of people outside the ITensor development team, and we do not always understand their inner workings in detail.
But taking a quick look at that code, it appears that QNA and QNB are how you set an initial state. I believe that setting these creates an initial state which is a product state where each site (of the two sites) has a state defined by the quantum numbers QNA, and QNB, where these quantum numbers are defined through the IQIndex sites (but unfortunately the description of this part is vague, I agree, so you'll have to read the code or ask the code's developer).
He does describe the usage somewhat in the readme file here: https://github.com/hoihui/itebd
The line "Sys(ampo,2,3)" is defining an object of type "itebd" and calling its constructor function, which takes an AutoMPO object and two integers. The itebd type is defined as "struct itebd ..." in the itebd.h file.