To assign tags to an area of responsibility:

Double-click an area in the sidebar.

In the sheet, enter the tags you want to assign to the area.

Click OK.
Things lets you assign tags to areas of responsibility.
If you assign a tag to an area of responsibility, all to-dos and projects inside that area inherit that tag. Therefore, if you later filter a list by that tag, those to-dos and projects will show up even if you haven't explicitly assigned that tag.
Double-click an area in the sidebar.
In the sheet, enter the tags you want to assign to the area.
Click OK.