Generally speaking this is how you determine convoy zone or convoy route ownership:
At the end of each resolve combat phase (Japan, UK and US), check each convoy route and convoy zone.
If no nation has combat vessels (other than submerged subs and transports) in the sea zone then the convoy zone does not change hands. If both Japan and the Allies have combat vessels (other than submerged subs and transports) in the sea zone, the convoy zone does not change hands. If only one nation has combat vessels (other than submerged subs and transports) in the sea zone, then that nation takes control of (or liberates) the sea zone.
Here are a couple of examples:
On J1, Japan attacks sz38 and eliminates the US fleet units there with naval and air forces - the naval forces include at least 1 DD, AC, sub or BB. Japan captures sz38 and the Philippine Convoy because, at the end of J1 combat it is the only nation with combat vessels in sz38. If they also capture the Philippines with an amphibious assault, Japan can claim the 3 IPCs for the Philippines.
On UK1, the UK sub from sz27 (which survived a Japanese attack on J1 by submerging after the first round of combat) moves into sz38 and immediately submerges (substalls) under the Japanese fleet. At the end of UK1 combat, the UK sub is still submerged (it does not surface until Phase 8 of the turn) so control of sz38 does not change as Japan is the only nation with combat vessels (other than submerged subs or transports) in sz38. The UK sub surfaces during Phase 8 of the UK1 turn.
On US1, the US undertakes various attacks but can’t get any units to sz38. At the end of US1 combat, both the UK and Japan have combat vessels (Japan fleet and 1 UK sub) in sz38. There’s no combat in sz38 because it is the US’s turn. Since both Japan and the Allies have combat vessels in the sea zone, control of the convoy route does not change. It remains with Japan.
On J2, Japan moves all of its combat vessels out of sz38 to attack various other targets. The result is that at the end of J2 combat, sz38 is occupied only by the UK sub. Control of the convoy route changes to US (it is liberated by the UK for the US) and Japan cannot claim the 3 IPCs for the Philippines because it does not control both the island and the corresponding convoy route.
Of course, the Japanese would be foolish to not attack sea zone 38 on J2 because it needs those 3 IPCs. So, instead of the example in the previous paragraph, Japan attacks the UK sub in sz38 with 1 DD and 2 Fighters. The UK sub is hit by the fighters (with the DD spotting for them) but in return fire, the DD is sunk (the DD is the only valid target for the sub). At the end of J2 combat, sz38 is empty. Both the Japanese DD and the UK sub are dead. Applying the same rules as before, the sea zone is now empty and therefore it does not change control. Japan retains control of the convoy route even though it doesn’t have any units in sz38.
I hope that helps. If you have any other questions, I’d be happy to try to answer them.
SS