AT&T System 25 - Manual (Page 7)

Data Dial Code (DDC)
When your System 25 was installed, Data Dial Codes (DDCSs)
were assigned to data terminals. If you have a data terminal
associated with your System 25, you will receive separate
information regarding its use.
Personal Dial Code (PDC)
Personal Dial Codes (PDCs) were assigned to telephones. In
most cases, your PDC is your extension number. If a telephone
is shared by two or more people, each person may have been
assigned their own “floating” extension number (FPDC).
Additional Information
See your System Administrator if you have any questions or
would like additional information about your System 25 features.
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