Does ECO openings classification have an established more detailed specification - by sub indexes? For example an ECO index B33 can be used to describe opening theory that barely fits into a thick book. Is there an established system to classify variations further? I have seen examples of indexes that look like B33/01, or B33/22 but I wonder if those are well documented anywhere?
A particular ECO index might split further into sub-indices like B33a, B33b, B33c, etc. The number of such sub-indices varies (maybe it goes up to y, or only to t, or ...), and some indices don't have any sub-indices. As for a place where these are well-documented, the free database program Scid does a great job. It automatically tells you what ECO classification you are in as you view/enter a game. More than that, though, you can bring up its ECO Browser that offers a summary of the ECO code you're in if you want to see the sub-indices laid out.

For instance, when I opened Scid and entered the moves 1.e4 c5 2.Nf3 Nc6 3.d4 cxd4 4.Nxd4 Nf6, the info panel below the board indicated that we had entered into ECO index B33a. That text is clickable, and doing so brought up Scid's summary for ECO B33, complete with variation names and clickable lines (which will bring the final position up on the board), as seen in the following image: