These systems allow to attenuate separately UpLink and DownLink.
It is possible to separate in frequency (FDD, different frequency bandwiths for UpLink (UL) and downlink (DL), or in terms of signal direction (TDD, same frequency bandwidth for UL and DL
As a reminder, in wireless networks, UpLink is defined as communication from User to Base station and DownLink as communication from Base station to user.