If a person gets a "Proceed immediately to..." assignment, which is a "Sink" objective, those will generally be a minimum of 2500 to 10000 tons. The 10000 tons are usually designated by tonnage with something like "Sink 10,000 tons of enemy shipping" or similar. The others might not be. In all cases, a "Sink" assignment can be accomplished anywhere in the SH4 world, where you can find enemy traffic. When you have one of those "Sink" assignments as a secondary that kicks in upon arrival on a Patrol assignment, those might be as low a 1 ton, or up to 1500 tons, all generally obtainable with the sinking of one ship. No penalty of course, if you do not sink any shipping, other than not earning the points for completion of the objective. But you can still sink anything anywhere, though you are confined by the Patrol requirements still... it is best of course, if you play with duds enabled, and get to experience the frustration of several hours of set-up work ruined by BurOrd...
P.S.: I think the Pro version allows several sandboxes for the kiddies to play in... or would they be "old legacy kiddies"?? and it's 4 gig, not 2... lol