More colloquially, this is about the care and feeding of deacons. A deacon isn’t a free agent; he must be connected to a faith community, not of his own making:
18. Any deacon who is not a professed member of a religious family must be duly enrolled in a diocese.
Most deacons have work in the world, but this statement speaking of sustenance should be interpreted along the lines of spiritual sustenance. #20 says keep the man’s family in mind, too:
19. The norms in force with regard to caring for the fitting sustenance of priests and guaranteeing their social security are to be observed also in favor of the permanent deacons taking into consideration also the family of married deacons and keeping article 21 of this letter in mind.
20. It is the function of the Episcopal Conference to issue definite norms on the proper sustenance of the deacon and his family if he is married in keeping with the various circumstances of place and time.
Thoughts, comments, questions, concerns, especially from any deacons in the readership?
Catholic Sensibility 
The view of the deacon’s secular employment has developed a good deal beyond this particular text. Now his job is considered a part of his ministry, and ordained presence in the workplace that should bring the gospel to those he works with by his example. Diaconal ministry extends to co-workers who are in any way oppressed or suffering from sickness or distress. The concept of the “worker priest” was shot down some time ago, but the idea seems to have met its fulfillment not in the priesthood, but in the diaconate.
Often when people learn that the deacon has a job and provides for himself and his family without assistance from the Church, puzzlement ensues: “So you are a part-time deacon?” The answer to this question is that the diaconate is full-time; just not all of our ministry takes place within the confines of the parish. The extent of the deacon’s ministry within the parish depends upon the time allowances of the deacon’s life ministry, on the margins and gray areas between Church and society.