Mipmap levels are generated automatically when this flag is set.
When a render texture is with mipmaps (useMipMap), then by default rendering into it generates all the mipmap levels. The mipmap
generation happens automatically only after rendering into this render texture; when active render target is switched to some other render texture.
If you want to render into mip levels manually, or control when mipmap generation happens (via GenerateMips), set this variable to false.
Default value is true.
Additional resources: useMipMap, GenerateMips.