Second Class or Third Alt Cabins on G Deck

[Harry Lee]

I was just looking on plan of Titanic, and I saw on G Deck, some Alt 2 or 3rd Class Cabins, why did these cabins have letter G and two digit number, unlike Aft and Forward areas which had a dash and 3 digit number, what the difference between these cabins?

 

[bee]

2nd class, like first class had cabins labeled with deck letter. 3rd class had numbered cabins in lettered areas because due to the watertight bulkheads there were adjacent areas of 3rd class cut off from each other. Separate staircases from decks above. The versatile area was numbered like 2nd class but had also the 3rd class label 'N'