Dial tone on the PSTN E side pair is jumpered to the FTTC 'in' pair, which for cab lay out reasons, and I suspect ease of understanding, is a D-side termination. Off to the DSLAM and reappears on the FTTC 'out' pair, which is an E-side. Then connected to the PSTN D-side and off the the punter.

The frames in the main exchanges work in a similar fashion, with the two sides of the frame being designated E and D. Exchange and distribution.