A Territory is a geographical region such as a country, group of countries or worldwide. Rights to music are often split between different territories - i.e. Record Company A has the rights to the Release for North America, Record Company B has the rights to the same release for the rest of the world. Therefore, the same release will be credited to different companies in different regions, and may use different or the same identifiers.
An Album, Single, Compilation, EP or any other grouping can be a release group. A release group is the summary terms of all of the release versions - we think of an Album as a singular release, despite the fact that it may be made up of Digital, ...