Welcome Shazza, they usually take the sentinel node which is the one closest to the breast and yes it it to find out if its thinking of branching out to somewhere else. If the node pathology is negative for cancer that's great but if it tests positive it means they will do another op in the arm pit to take other nodes to test. That's the bit they never told me and I was totally unprepared for the possibility of it. Knowledge gives you preparation time to cope. Here is a search engine to find your local postcode one near you.
https://www.mcgrathfoundation.com.au/OurMission/OurNurses/FindANurse.aspx?close=y Contact her and she will be your advocate in finding out lots of things for you and for support. <3