This block (as well as any other channel block) has a connector on the rear side through which any number of “extra” blocks can be installed to add features to the channel: extra effect send/returns, extra submix buses, vuMeters, recorder/loopers, etc.
Some of the extra blocks connect between them forming rows in order to mix their signals (like for effect sends). These blocks also connect to their respective docking block to send the mixed signal to the master mix.