Additional texture coordinate.
UVs usually use Vector2 format. Vector2 uses two 32-bit floats; 8 bytes in total.
This vertex attribute maps to the TEXCOORD3 semantic in HLSL. This UV channel is also commonly called "UV3". It corresponds to Mesh.uv4.
Additional resources: Mesh.HasVertexAttribute, Mesh.GetUVs, Mesh.SetUVs.