There are some descriptions in the manual:
the WMB format is compiled from a WMP level by WED's map compiler. It's basically a collection of lists for the level entities, blocks, textures, lightmaps, and so on. Of the 20 lists, only 5 are used for a simple level. The rest are 'legacy' lists that were only used in old format versions (WMB1..WMB6), and 'bsp' lists used only for BSP tree levels. They can be left empty. An empty list has offset and length both at 0.
typedef struct {
long offset; // offset of the list from the start of the WMB file, in bytes
long length; // length of the list, in bytes
} LIST;
typedef struct {
char version[4]; // "WMB7"
LIST palettes;// WMB1..6 only
LIST legacy1; // WMB1..6 only
LIST textures;// textures list
LIST legacy2; // WMB1..6 only
LIST pvs; // BSP only
LIST bsp_nodes; // BSP only
LIST materials; // material names
LIST legacy3; // WMB1..6 only
LIST legacy4; // WMB1..6 only
LIST aabb_hulls; // WMB1..6 only
LIST bsp_leafs; // BSP only
LIST bsp_blocks; // BSP only
LIST legacy5; // WMB1..6 only
LIST legacy6; // WMB1..6 only
LIST legacy7; // WMB1..6 only
LIST objects; // entities, paths, sounds, etc.
LIST lightmaps; // lightmaps for blocks
LIST blocks; // block meshes
LIST legacy8; // WMB1..6 only
LIST lightmaps_terrain; // lightmaps for terrains
} WMB_HEADER;
