I'm going for a more gritty/realistic game, and so I'm having my players be careful about their carrying capacities and weight. While helping a new player build a character, I noticed that the explorer's pack they received is overweight if the waterskin is considered to be full and the rope isn't counted. It notes that backpacks can have things strapped to the outside, but does this weight count toward the total weight a backpack can carry? If stuff strapped to the outside doesn't count toward capacity, is there any limitation besides imagination to just how much could be strapped to the outside of the bag?
The weight limit for packs/bags/etc is for the contents of the pack/bag/etc. A container can only hold this much. There is no explicit limit to how much can be tied to the outside of a pack, besides a character's total Carrying Capacity, and objects attached to the outside do not count against the weight limit for objects placed inside.
Beyond the fact that you are asking about a level of granular detail that the rules to not cover, I suggest that you do some research into what a loadmaster has to do for a truck or an aircraft. There are three primary challenges to address:
Order of loading.
I had to address the cube problem a lot when I worked during the summers for a moving company. It all had to fit, and the truck could carry more if it were just the weight being carried.
Each of these constraints can be applied in a micro sense to the pack that an adventurer carries, though "what has to come out first" is the most easily planned for.
- Some things aren't that heavy, but are a bit awkward to carry. The front door of your house/apartment is an example.
- Some things fit into a small space but are very heavy: a cubic foot of lead being one example.
- Some things you carry and need to get to "in a hurry" and other things you carry and can get to at your leisure.
As an illustration, look at the seventh edition of the Boy Scout Handbook, pages 250-251, on a properly packed Yucca pack. (If the link doesn't take you to that page, flip to those pages). That demonstrates a model of "how much stuff fits into a certain sized pack" without too much complexity.
To scale this up a bit, a US Army Infantryman carries about three times that much in his pack: partly due to the density of the contents (ammo is dense and heavy) and partly due to the size of the pack. The cube is a bit more than twice that of the Yucca pack, but when all of the items are added up, the density increase is non trivial. The soldier's pack will typically top out at 80 pounds, sometimes a bit more. That doesn't include the weapon carried.
With these practical points provided, you may be able to better visualize the limitations of a back pack full of gear. If nothing else, once beyond the weight limit of the pack, now and again roll a saving throw versus straps breaking. Pick a DC: I've had straps on a pack break. Not a good thing.
It's up to your DM to decide if what you list as included in the character's "backpack slot" is reasonable. I'd suggest that the weight limit is an abstract measure of how much kit you can have in that "slot", whether it is inside or outside the pack is just descriptive. The only real game impact is down to what happens if the character doesn't have it's backpack. This is both positively in terms of encumbrance but also in terms of what kit is missing if you have left it behind.