Nuke Tutorials - and the logic of node based compositing

Well from the tutorials I've watched so far, the logic of node based compositing seems pretty straightforward; much like the rigging hierarchy in the outliner in Maya... very satisfying and clear cut...and I really like that it's very visual...

I am intrigued to know more about the merging of A and B inputs, and why there are additional 'A' input options, such as A1, A2, A3 etc...rather than just utilising the alphabet in chronological order...

Really looking forward to the Nuke class tomorrow, where more will be revealed! In the meantime I discovered that Nuke came out in the early 1990's and was the 'New Compositing' software...hence the name 'Nuke'