TX START

From ZDoom Wiki

TX_START is a marker that is used in conjunction with TX_END. Any graphic in either Doom or PNG format that is between these markers is immediately treated as a texture/flat. Sadly, it has some downsides. 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 and Doom Builder.

If you are using ZIPs as WAD replacement then you can simply use a directory called "Textures".