Page "D4 framing standard" Paragraph 6

So, why is the T1 Superframe framing format commonly called D4 Framing?

There are may iteration of the answer but the most common by me over the years was that the abbreviations for Superframe ( SF ) was in conflict with the sill commonly used technology of the day, Single Frequency Signaling ( SF Signaling ).

SF Signaling was the method of Inband circuit control before the advent of the Extended Superframe ( ESF ) framing format.

The ESF T1 framing format allowed for the signaling information to be carried out of band and not as an Inband tone.

Consequently, the D4 channel bank was in deployment and so the Superframe format became associated with the D4 Channel Bank, or D4 Framing.