• We have updated our Community Code of Conduct. Please read through the new rules for the forum that are an integral part of Paradox Interactive’s User Agreement.
An Area encloses several provinces (e.g., Eire has the thirteen counties of Ireland) and a Region contains several areas (e.g. Britain has Eire, Scotland, Wales, North_England and South_England). It looks like your guess is right about Seazone; I know when I ship troops out of Sligo they go first into Donegal Bay, and Sligo's Seazone is 908, and province 908 is Donegal Bay.
 
Event scripting, I think. There's an English Melting Pot event which only fires for the North_England and South_England areas.

Oh, and maybe AI purposes too, like in EU2, though here the AI is hardcoded rather than scripted.