In the actual gameplay, no convoy ships are involved. Actually, in several missions you have to equip your "escort ships" with additional storage, the "
drum cannisters", which strongly suggests that the shipgirls themselves are the ones carrying the cargo with no assistance of merchant or cargo ships.
Moreover, if the expeditions were "escort missions", you could lose a number of protected ships, increasing or reducing your mission rewards. Such mechanic does not exist in the game. It is an inconsistency in the game, actually. The expeditions are duration-based: once the indicated time has passed, your fleet is back, and you get the reward. Some missions cost fuel and ammo, which suggests combat, but the fact that shipgirls come back from an expedition unharmed suggests that it is actually some kind of shore bombardment with no return fire. That would mean that the "expeditions" are actually some kind of "supply raids", and you don't send civilian ships to raid missions.
I know this is a rather long time to be replying to a post, but you're working under the assumption that the Expedition descriptions are vague and undefined. They are in fact not.
Expeditions
This was provided to me years ago upon request, with translations of the descriptions in the Expeditions. Each of the Expeditions not only have a specific title, they also explicitly mention what they entail.
For example, Expedition 5 is "Maritime Security Missions" with the description "Escort a convoy out of our waters to ensure it's safety!". Note that Expedition 5 only has a 5 Fuel requirement with no Ammo expenditure whatsoever, thus implying that it is an escort mission and not a raid as you assume.
Expedition 6 on the other hand is "Anti-Air Exercises", with the description "Prepare against an enemy carrier-based attack in a special training session!". In accordance to the description, the expedition has a 5 Fuel and 2 Ammo expenditure, indicating the use of live fire.
Note another convoy escort mission, Expedition 9 which is "Tanker Escort Duties" with the description "Escort a Tanker Convoy carrying a great supply of fuel back from the oil fields!", which also has only a fuel requirement with no ammo expenditure.
And of course, you have stuff like Expedition 10's "Attack Force Recon Missions", described as "Utilise a Seaplane Reconnaissance Aircraft Carrier to gather Intel on the enemy fleet!".
Also note another expedition, number 21 aka "Northern Rat Transportation Strategy", described as "Send out a Torpedo Squadron at night in an operation to transport supplies to allied ground forces in the North! Use Drum Canisters to carry rations and other supplies!". Note that this is one of the few expeditions that directly mention having your own shipgirls carrying the supplies specifically, whereas all other convoy escort expeditions do not, giving the implication that our shipgirls are not the ones carrying the supplies in the other missions.
It's clear that Expeditions cover a wide gamut of activities, including Naval Reviews, Decoy Operations, Sneak Attacks, Patrols, Defense Missions, Technology Exchange, etc. In this sense, Expeditions are pretty much just missions that you don't have direct control over unlike Sorties.
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