The law concerns a tax applied to all merchants and people who received fees for their work which is known as collatio lustralis (or chrysargyron in Greek). See Seeck 1900.
According to Delmaire 2005: 192n1, because the law is addressed to the proconsul of Africa, the reference to vita sanctior might mean that the tax privilege should apply only to the Catholic clerics and not to the Donatist ones. Delmaire also propose that the given date is maybe the date of posting the law in Africa for public display and in fact it was issued year earlier (similarly as 13.1.18 also about the tax).