While the boundaries of official structures (countries, administrative regions, etc.) are defined quite precisely, non-administrative regions exist. It is more difficult to define their boundaries because they are not tracked by some official body, but since they have names and people use them, they should appear in the database.
boundary=usage The fuzziness of the boundary could be denoted by using the fuzzy tag. For the largest areas, it will usually be better to use a multipolygon relation. However the fuzzy tags should remain on the individual ways in the relation since the fuzziness can sometimes vary.
Note that a few international borders (as well as some administrative border within a country) are also fuzzy, e.g. when the boundary is in fact somewhere in an unoccupied desert, or there's no defined agreement on an exact maritime boundary. Despite of this, the defined name is not just a "usage" name, there is no doubt on the populated terrestrial area about its official name. In that case, the boundary should still be defined with boundary=administrative, but the fuzzy part of the boundary should be tagged on the individual unprecise way.
This is part of a group of proposals to include areas that are not easy to define accurately:
On classical maps the areas will not be rendered. However the general shape should be used to correctly render the name of the structure. It can be spread or extended across the area if it covers a large part of the display. If the feature covers the entire display, the renderer could not display the name at all, since it would be obvious.