On Jan 20, 1:34 pm, Mxsmanic wrote:
What determines the boundaries between airspace managed by an en-route
ATC center and an approach or departure center?

There is no fixed boundary between approach and center airspace. It's
usually defined by altitude though. Here in CT, once you climb past 15k
or so you get handed off to Center. I've flown VFR up to Maine below
10k and talked to Center - it's probably based on letters of agreement
over who owns what airspace and at what altitude. When at or above 18k
(Class A airspace) you're talking to Center no matter where you are,
although there may be exceptions to this rule in some places