in which situation a new Inventtransid is created

hi. I need to know how/in which situation a new Inventtransid is created.
There are 2 lines, with the same inventtransid & same inventbatchid, but 2 different quantities. I assume in this case there should be 2 different inventtransid’s as well. So, the question is what is the reason for having only 1 inventransid.
Thanks.