Channel packing
Stuffing several unrelated grayscale maps into the R, G, B and A channels of one texture to save memory and texture fetches.
A metallic-roughness workflow needs separate ambient-occlusion, roughness and metallic maps, but each is a single-channel grayscale image — wasteful as three full RGB textures. Packing them into one RGBA (the common ORM layout: occlusion-roughness-metallic) cuts memory to a third and, more importantly, lets a shader read all three in a single sample instead of three.
The catch is that such maps should not be compressed or mipmapped as if they were colour: block compression and sRGB curves assume correlated channels, and uncorrelated data trips both. Teams use a linear format or accept some bleed. A normal map's two meaningful channels are sometimes packed alongside, with the third reconstructed in the shader.