A license is what most, if not all heavenly beings, have. In the series, we have encountered licenses on goddesses, demons, and one Earth Spirit. The licensing system is a parody of the Japanese engine licensing system (as Mr. Fujishima likes motor sports).
Gods usually introduce themselves with their name, class, category, and whether they're limited/unlimited.
The class number reflects exactly how much power you have access to.
At first it was thought to be how much power you innately have at any given point, but recently through Urd's First Class Examination it was revealed that with a specific license, a goddess is actually restricted by their class once their power exceeds the class's limit.
So, while Skuld has little to no power to begin with, she's a second class goddess. However, Urd, who's also second class, has tremendous powers inside of her (probably due to age and experience), but she is limited by a limiter, which can be removed by a first class goddess. First class is the highest attainable class, as far as we know. The lowest class currently known is third class.
The category, or type, reflects the job of the goddess. A Category One would be a private license, and would most likely be involved in administering Yggdrasil or participating in private matters in Heaven. A Category Two is a commercial license, in which they make contact with human beings via agencies. Two goddesses with a Category Two license are Belldandy working for the Goddess Relief Office and Peorth working for the Earth Assistance Agency. There exists at least one other category called Special Duty, the Valkyries. Lind is a part of Special Duty, and Special Duty deals with combat.