TX_START is a marker that is used in conjunction with TX_END. Any graphic in a supported image format that is between these markers is immediately treated as a texture/flat. Sadly, it has some downsides. These textures will override textures of the same name in the flats or walltexture namespaces. TX_START/END textures and flats cannot be "multi-patch" or scaled bigger and smaller like the usual TEXTUREx textures. They can still be warped, however. The only level editors that currently support its use are DeePsea, Doom Builder, and SLADE 3.
If you are using ZIPs as WAD replacement then you can simply use a directory called "Textures".