The Whitefish is organized by sections, which are usually representative of a single discrete facility. For instance, the section labeled A10-3 represents the Captain's Quarters. Sections vary wildly in size, and some facilities may take multiple sections. A section's label is formatted so, where a template might be XNN-N(#NNN):

X indicates the wing that the section is located in, either A, B, C, D, or E.
NN indicates the level that the section is located on. There is a total of 11 levels from 00 to 10; single digit numbers are preceded by a leading 0, e.g. 01, 02, 03. Most wings do not include all 11 possible levels, and will exclude the bottom or lower levels.
-N indicates the column that a section is location in. Each wing include 5 columns, from -1 to -5.
(#NNN) indicates the block number. It is placed in parentheses because not all sections will be separable into individual blocks, such as the mess halls.

Block numbers are mostly unused due to the complications in recording each and every block on a ship fit for several thousand people. They are mandatory when presenting the block number of a hiveblock (apartment/personal living space), however. Each section that is officially named "Residency" section has exactly 200 blocks, no more and no less. When listing a hiveblock's section label, this means that the block number must be included for clarity, such as "D03-2#99".

If you are confused by some of the contents of the mapbook, the Setting Document's Navigation & Residency section might help.