You should download the condensed rules manual from here. It's not 100% up to date (1.08.09 vs 1.09.02) but it's a hell of a lot better than the now often useless official manual.
In short, HQ buildup is almost completely different now. Any HQ can do it as long as it has at least one movement point left and is not overloaded (for obvious reasons...). All units attached to it that have at least 25% of their MP remaining and are within range will be affected by the buildup. When you clicked on your HQs and some said 1 unit and others 3, some of those corps must have had units either out of range or without enough MP. Unlike before, there is no immediate cost in damaged or destroyed vehicles and all remaining MP are removed from the affected units.
Basically during the logistics phase they now get a higher priority for supplies, with halved penalties (from leaders and the rail multiplier etc), but at the cost of doubled truck usage and fuel usage. They will generally always therefore fill up well above 100% of their needs and also benefit from halved penalties on leader checks for their MP, so they will probably also have most of those. The HQ also tends to stock up a fair amount of dumps too. In return, you will damage quite a few trucks, but not like before. Now the function is part of the normal logistics system and more or less works by giving a high priority to the affected HQ. In other words, it's a bonafide, good old fashioned hack.
< Message edited by RoflCopter4 -- 2/1/2017 6:30:04 AM >