region group
The tier a region occupies in the worldwide hierarchy, which is what makes places in different countries comparable.
A region group is the tier a region occupies in one hierarchy that runs from continent down to land parcel: continent, country, state, county, city, first-, second- and third-level city district, block, land parcel. It states how far down a region sits and therefore what may contain it, so a county belongs to a state and a city district belongs to a city.
The tier means the same thing everywhere on earth. That is the whole point of it: a first-level city district is the first tier below the city whether the city is in Germany, Brazil or Korea, which lets the two be compared without knowing either country's vocabulary. The hierarchy is enforced, so a region cannot be saved under a parent the tier does not allow.
What the unit is called locally is the region type, and it is a separate choice. The same tier carries a different word in each country, and the same word appears on several tiers: this catalogue holds "district" on five different tiers and "county" on two. Neither may be inferred from the other.
The tier also says nothing about how the area came to be drawn. A city can be divided politically, statistically and cadastrally at the same time, and all three sets of areas sit on the same tier; the division basis is what separates them.