I think of it not just in terms of mission, but in duration there of. For example, EDC is <24hrs and letting you mind your own business with some provision for dealing with the really odd stuff.

A PSK is 24-72hrs, bare bonesing it. Don't need food or much shelter, unless the weather really sucks. If you've got some kind of wind/vapor barrier, and a way to start a fire, you are decent in temperate or warmer weather. It's job is to keep you alive until you find your way out or get found

A small BoB/ditch kit/get home bag, I'd say 24-72 hours with lousy weather becuase you've added water, food and shelter. It has a lot of cross over with the PSK, with the big difference being the PSK compliments a lot of gear while a GHB is a PSK and the outdoors stuff most of us don't normally have on us as we have become urbanized. My ditch kit carries core equipment, so if I have to abandon BoB for some reason I'm not completely out of luck- it might not be the gear I REALLY want, but I can make it work. It's mission is to get you home, or at least get you most of the way there, if you don't have most of your gear, OR give you the ability to hole up for more than one night.

A classic BoB is more for 48-100 hours- Big BoB might got up to 150, 175 hours, but that bruiser is about the limit IMHO. Self contained for everything but water and air for that period, and maybe fuel, it should be backed up by at least a PSK or (better) a ditch kit. A BoB gets you were you were headed if the car breaks during an evacuation, or if you have to run away RIGHT NOW.

(Now, when I say BoB is backed up by a ditch kit, that doesn't mean they are separate, but rather BoB adds to the ditch kit. I'm good for 24-48 hours with my ditch kit in terms of food, and a lot longer than that with my fire making and water purification systems. BoB just has more of everything and real shelter, giving me a total of 120 or so hours.)

As for the I'm Never Coming Home bag, those who cross post with me to Zombie Squad are about to scream "burn the heretic". I think the INCH kit is a fallacy, the "run away and live like a cave man" fantasy in solid form. I know this is a point of semantics, but semantics aren't only about saying exactly what you mean, but the effects of phrasing on perception. The idea you can fill up a bag with everything you need to never have a place to call home again unless you build it yourself out of logs or adobe or whatever is what it implies to me. It also implies that you have no plan other than running away and making it up as you go along. Now, if that isn't what you mean, I'm not calling you crazy, I'm asking what you mean and to do so I have to say what I mean.

BoB is self contained for 4-6 days, but it doesn't self destruct just becuase you are out of clean socks and food. BoB should have documents in it, so that if the reason you had to run away right now ate the house and all your records, you can restart. You should have hygiene and water purification and food procurement methods in there. You've got shelter and spare clothes. So in effect, BoB will get you through anything within reason, even if you do have to get creative and have space between your navel and spine. But at this point you are a refugee, get used to it.

By day two or three you should know if you even have a home to go home to. The minute you get word that you are now the proud owner of charred/flooded/radioactive land that has no inhabitable structure on it, you need to go to ground if you are in a reasonably safe location and start thinking about the next step. That next step should not be running off and "living off the land". If you have a never coming home situation, you need to be thinking about friends and family, places where you crashing for a few weeks or a few months won't cost you allies. You need to be thinking about a job if your old place of employment is gone, even if that new job sucks. But even if you are living in your car and bathing in the sink at work, fine. You are still you and you can prove it. You are still alive. BoB did his job. What you need at that point is a new place to live, and possibly a new job, neither of which fits into a backpack, or a car, or a really neat cart. If you want to go into TEOTWAWKI/PAW sillies, then you need to make yourself useful before the locals put you to the road or on a spit. But honestly, it is no different than having your home burn down, or having your employer toss you out the door or lock that door. There are mechanisms in place, and if your pride is too heavy to allow you to use them, then you need to rebuild your kits with a little less pride.

I guess the point I'm trying to make is this: the semantic difference between INCH and BoB is how honest are you in your planning? If you think you need to be out more than 3-4 days, then you might be in the market for a bigger bag or a supplement like the bug out carts we've seen. But if you aren't planning on having anything to come home to, it tells me you haven't planned it out very well. Or you need better friends.