waveguide port in CST

Yet another question: what is the reference plane for a waveguide port? You can specify a "distance to the ref. plane", for -they say-a correct deembeding os phase info for S parameters. I modified that distance and I noticed that the phase is indeed changed (with frequency) But I don't understand how/where should that reference plane be defined for a specified structure (for my TEM cell).
Thanks in advance!
C.

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