The fix layer is a non-scrollable, tile-based layer with transparency which has the highest priority on the display (it always overlaps sprites). It is most often used to display text or HUDs like scores and health bars.
As only 12 bits are reserved to address fix tiles, tilesets can only contain a maximum of 4096 tiles (128KiB) without any kind of special bankswitching (see below).
Fix tiles are 8*8 pixels, they're stored in the S ROM, or in FIX files before getting loaded to PSRAM in CD systems. In MVS systems, there is an embedded fix tileset stored in the SFIX ROM which can be swapped with the cartridge's one.
The tiles graphics are 4BPP like sprites tiles (15 colors, index 0 is transparent), each can be individually mapped to one of the first 16 palettes (4bit palette index).
See Fix graphics format for more details.
The map is in VRAM starting at address $7000. The area used for display is 40*32 = 1280 words in size, but only 40x28 tiles are drawn in NTSC mode (2 hidden lines on the top and bottom).
Most games don't use the leftmost and rightmost columns, as some CRT screens can cut them off. SNK recomends only using a centered 38x28 safe zone.
Each map entry in VRAM uses a 16 bit word:
|Def||Palette number||Tile number|