There are a lot of variables that go into that... Notice on the Menu, when you choose a new Campaign Career, that you have at least 8 dates to choose from, some mods have more. Notice also, that for each of those dates, there are at least two bases that you can play from. From each of those bases, you can have anywhere from 2 to 10 submarines available to play. After that, the Campaign variables take over, and you might get any one of a number of missions, mostly based on chance, but also time frame, and whether or not you've done a particular mission or not... In the Data / UPCData / UPCCampaignData folder are the CareerStart.upc, which is like the "menu" you'll see after choosing a new career. Flotillas.upc controls the boats and bases, and assigns a code for missions, while PatrolObjectives.cfg then takes that code and pulls a mission for you to use out of the Data / Campaigns / Campaign/ PatrolObjectives folder.
In Stock, there are 233 mission folders. In FotRSU, there are 309. Out of those, any one choice of base / sub / timeframe might have 3-6 missions to choose from. For first missions, Pearl Harbor boats are either going to Empire Waters, or the Marshall Islands. Manila boats are generally going to be somewhere around the Philippines, but might also go to Formosa or China. You can see those as "Objective 1" in the Flotillas.upc file. Get the code from that, and then look up that code in PatrolObjectives.
If I'm remembering correctly, you should find one of three missions for Empire Waters, and one of four for the Marshalls. I cannot remember the Manila assignments off the top of my head, but like Celebes Sea, Lingayen Gulf, Luzon Straits, Formosa, and a few others, but I don't remember how many of each...
All of that said, what exactly are you trying to do? Are you wanting a particular mission? A mission not working correctly for you? Stock will sometimes send you into Hiroshima Harbor, off the Inland Sea, and that might be just this side of a suicide mission, and would not have been done in real life...
