DDR2 SDRAM Controller with ALTMEMPHY signals

Hello,

I am using the DDR2 SDRAM Controller with ALTMEMPHY ip core. I received a reference design that I am basing my design from. What is the external_connection signal? In the reference design it is not hooked up to anything or exported. This produces a warning. Can someone tell me what the signal is for?

The memory conduit is what is exported and connected to the sdram.