My take on the terminology - only mine hope no one takes offence.

Micro: Single function - no larger than a tic-tac box. Included here are FSK (Fire starter Kit), SAK (Swiss Army Knife), KFK (Key fob Kit) - but not the MKK (Mega-Keychain Kit) that last is multi-function so doesn't fit here.

PSK (Pocket Survival Kit) - fits in a pocket. Many use the Altoids tin, some use a slightly larger Tobacco tin, some use a variety of soft-sided wallet sized pouches or even the Mil-Surplus FAK belt pouch or the like and some use novel approaches such as MKK. Point here is that this is the smallest kit designed to live out of if need be. This should contain enough tools of sufficient quality that you could construct shelter, find and purify water, stay warm, stay fed, and signal for help and last for a minimum of 3 days up to indefinately. This is the first level of redundancy and is always carried. The size is constrained in both directions - it needs to be small enough that you are not bothered by carrying it everywhere no matter how you are dressed or where you are going and it needs to be large enough to be complete. This size constraint leads to many different compromises each based on personal taste, skill, risk profile and health / well being. It must be complete and you must be willing and able to carry it everywhere. An example of a compromise for space and weight savings that I have heard of is to carry a metal pop bottle cap filled with vaselline soaked cotten and wrapped in a ranger band as your only fire start. This saves space over many of the alternatives but lacks in redundancy and relies upon the suroundings to complete it. (The pop bottle cap becomes the bearing used for a bow fire drill - If you know then 'nuff said, if you don't then get a mini-bic)

PFAK(Pocket First Aid Kit) A single function pocket sized kit focesed on bandaids and medications. Many add this to the PSK for ballance - it makes both pockets the same size so you don't list to port too badly. Also allows a wider variety of stuff in your PSK since you won't need any room there for first aid supplies and allows a usable quantity of FAK since to squeeze bandaids into your PSK will always mean too few bandaids to be worth it.

NSK(Notebook Survival Kit) or the Maxi: The next step up in size and level of convenience. This size is too big to be comfortably carried everywhere. It will be described as always being nearby in the desk drawer at work, in the car while you are in the restaurant, etc. This kit may contain some shelter such as a trash bag or plastic poncho or silnylon tarp but it won't be with you if you have to stand and run or if you get dropped out of the sky or over board the cruise liner etc. I also include in this category my VSK (Vest Survival Kit) because in warmer weather I often take that off and leave it hanging on the back of the door or coat rack etc.

BOB (Bug Out Bag) Everything larger than you are interested in carrying around all the time in the car goes into this category. Some think of evac from somewhere to home and therefore have a smallis BOB that travels with them. Some think of evac from civilization to the safe-haven of the retreat in the hills and have a full-on expedition camping setup that lives at home next to the 55 gallon drums of gas, and water and the 5 gallon pails of dried food-stuffs waiting to be loaded into the off-road vehicle with hood mounted cannons and oversized tires.