The first letter is what you build : B for Base, C for Conduit, P for Power station.
The number is the basin number, it goes for 1 to 12 . It counts top to bottom, left to right. 11 and 12 are just for the power stations that are below the last basins.
"U" means upper.
So B10U is a base in the lower right basin, the upper slot (the one costing 3 gold).
And B10L would be the base below it.
For power stations, you could get something like P11_1 where _1 indicates which of the slots you take at place 11.
I don't know exactly how the _1 is determined.
Conduits would be something like C2B :
C for conduit, 2 for basin 2 (that is the second mountain basin) and B would be the second conduit slot in that basin (from left to right in alphabetical order)
Sources are labelled HA, HB, HC, HD (from left to right)
