Ever Made Your Way As Far into the Interior as Coruscant?
STAR WARS EPISODE II: Attack of the Clones 00:51:29
The definition of the Interior as its own region comes from the 2019 Galactic Explorer's Guide which has a nice simplified map showing the regions.
I actually prefer this way of showing areas of the galaxy, based off of a geometric circular grid rather than the "wobbly" borders on other maps.
If we compare the borders of the Interior to the maps we have from the Essential Atlas, the Interior could be further broken into the Negs, the Northern Dependencies, the Southern Core, and the edge of the Hyperspace Anomalies we now know as the Red Honeycomb Zone.