Sub-region
A sub-region is a section of a region that comprises a couple of areas. A sub-region has its own map showing those areas instead of just one area.
For example, in Moria its map is split in sections, or sub-regions. The map of a sub-region fills the purpose of guiding the traveller with better resolution than the map of the region can do. And they may display one "level" (floor) out of many different levels of a 3-dimensional region such as Moria. Etc.
An alternative description: a sub-region is the map itself. Or rather the info the map conveys, namely a couple of areas.