The various citations I'm looking at are:
Total Warfare, pages 136-137 (Under "Lift Hoists")
Total Warfare, pages 226-227 (Under "Mechanized Battle Armor")
Total Warfare, pages 260-261 (Under "Cargo Carriers"), as amended by the below errata
Total Warfare Errata v4.3, pages 43-44 (Under "Cargo Carriers")
Tech Manual, page 245 (Under "Lift Hoist/Arresting Hoist")
Apart from this collection of rules being scattered across four places in two books, it appears to be in conflict with itself in several ways:
1) The Lift Hoist rules imply the maximum amount of tonnage that can be carried is limited only by the number of Lift Hoists mounted on the unit. This is not a problem for 'mechs, as they're limited to two Lift Hoists (thus yielding the tonnage of the unit as the maximum, as stated in the Cargo Carrier rules on pages 260-261, potentially doubled for TSM). For Combat and Support Vehicles, though, it seems the limit is twice the unit's tonnage (since they can mount up to four Lift Hoists), thus conflicting with the limits on pages 260-261. It's possible (but unlikely) that the intent was to allow Lift Hoists to carry cargo separate from other "external cargo", which would theoretically make it possible to carry 300% the weight of the unit. So the question here is "How much external cargo can a Combat or Support Vehicle actually carry?"
2) The rules for "'Mech Lifting Capabilities" state a 'mech lifting 10% or less of its tonnage takes no movement penalty. This seems fine as an exception to the cargo carrying rules right up until you factor in Mechanized Battle Armor, as OmniVehicles take no movement penalty, just like OmniMechs, which would seem to imply Combat Vehicles (at least) should also be able to carry 10% external cargo without penalty. Question: "Can Combat Vehicles carry up to 10% of their weight as external cargo without penalty?"
3) The rules for "Mech Lifting Capabilities" state a 'mech lifting up to 10% of its tonnage takes no movement penalty, yet a non-OmniMech takes a -1 MP penalty for carrying Mechanized Battle Armor, apparently regardless of their tonnage. Question: "Does a non-Omnimech carrying a Battle Armor unit that weighs less than 10% of the 'mechs weight take a movement penalty?"
If this has all been sorted out in the BMM, great, I look forward to seeing the final version. If not, a clarification just might merit inclusion.