The ProTactic is quite flexible; it has the standard velcro dividers on the inside, but the outside is almost completely covered in loops which accessory pouches can be connected to. These can be purchased as add ons, but the bag comes with three when you buy it. They are a tripod support (with a seperate clip strap to lash it to the side or back of your bag), a water bottle holder (with a small hole in the bottom to let any spills or condensation drip out), and a small zippered bag which can hold a few snacks or small equipment. Also arranged on the exterior are a few small zip pockets, although these are quite small and you may not get much use out of them.
To fully open the bag, you unzip the side that rests against your back. This not only increases security, but it also means that if you have to put the bag down in a dirty location, the part getting dirty isn't then going to be pressed against your back. There are also three quick access panels which can be unzipped easily, one on each side, and an extra one at the top. This is invaluable for your camera body, or a lens that you often switch to.
Internal zippered compartments are confined to the back panel that you open to access the whole interior, there's no separate section for storing paperwork etc like my old CompuTrekker. There's a padded space for a tablet or laptop (up to 15 inches) as well as two smaller pouches with clear covers so you can see what's in them.
There is also a removable belt strap to help support the weight. Each side has an extra small zipped compartment, and the belt strap itself is removable. I didn't really use it on my older bag, and so the straps just got in the way. Being able to take it off entirely makes it much easier to handle. There's also a strap that goes across your chest.