The multi compartment models track loading and unloading of various different half lives. Many dive computers track 16.
I believe (but could be wrong) that the PADI dive tables are based on a single tissue half-life: 60 minutes.
To be more precise, the PADI table
repetitive dive calculations are based upon the 60 minute compartment. ALL compartment from 5 minutes and slower are used to determine the NDLs --- that's why deeper than 40' you will reach the NDL limit before reaching the highest pressure group, Z. (As a rough rule of thumb, the contolling compartment will the one with a halftime around 1/2 or 2/3 of the NDL time)
The 60 minute compartment was chosen for pressure group calculations because, with open circuit scuba, the other compartments will have reached their NDL before the 60 minute compartment does. So if you limit repetitive dives to keep the 60 minute compartment within bounds, then all of the other compartments are also within bounds.
This assumption of the controlling or limiting compartment being 60 minutes or faster is NOT true for sequences of 3 or more extraordinarily long dives, which is why the PADI table has the extra W,X,Y, Z rules that mandate a minimum 1 hour or 3 hour SI in some rare cases.
The USN tables were designed to be suitable for very long shallow dives, typical of surface-supplied divers working for extended periods on the bottom of ship hulls. In that sort of dive, the controlling compartment may be as long as 120 minutes, and that is what the repetive groups of the USN tables are based upon. This is why the USN tables are excessively conservative in repetitive dive calculations for open circuit recreational scuba profiles, even though the USN table limits for the 1st dive are much more liberal than the PADI table.
