ZDfB stands for Zertifikat Deutsch für den Beruf. It is set at the B2 level on the European scale of language learning and is intended for adult learners.
The vocabulary that is taught is aimed more towards words that would be used in an office or in industry, although at a moderate level. As such someone with ZDfB could make telephone calls or write correspondence, but it is not the highest level that may be required for more qualified jobs.
To hear a simple explanation and a short discussion in German, listen to the podcast:
(Press the “play” button to listen to the podcast)