Guide to Block Compression (BC1-BC7) of textures.
Using VRAM wisely is a vital part of impressive results in real-time rendering. BCn compression uses genius tricks to condense textures into VRAM. Selecting BCn compression for your needs makes your rendering stunning at optimal cost
https://dev.epicgames.com/community/learning/tutorials/Vxk9/unreal-engine-bcn-texture-compression-guide