What the differences between BDM1 spaces and [P1,P1] spaces?

For 3d simulations, we want to use BDDF spaces, but FreeFem++ do not have this type of spaces, only have RT03d. So we can see the definition of BDM1 or BDDF1 spaces is that the vector belongs to P1^2, so why we can not use [P1, P1] spaces to substitute? Or is there any open sources about how to construct the BDM1 spaces in FreeFem++?