this is deep into the grey area of the statistical curve fitting that is compartment models, does not take into account unpublished 'revisions' to the base profile by noted agencies, and is proposed by a complete newbie diver with only 13 dives in the log, a basic armchair understanding of deco theory, and software written from said understanding thereforth. consider the following more a validation of understanding, rather then a statement of fact.
To complicate matters, at least one agency (the big one) says certain safety stops are required.
i've only personally ran SSI, NAUI and PADI tables through as published ZH-L16[bc], but strictly speaking, all of their NDLs invoke a deco obligation ( deco ceiling / tolerated ambient pressure (TAP) of over 1.0 ATA) at the edge of the table around 80' to 100' fsw. with an 80' u-profile, NDL is 30 ~ 35 minutes depending on the tables in use; if you constrain your deco ceiling to 1.0 ATA, it bails into required ascent around 26 minutes in. thing is, even with the full 35 minutes BT, any deco stop incurred, is cleared by time you hit the safety stop depth during a nominal ascent of 33f/10m per minute.. but only just.
if you take base ZH-L16c, leave off the deco ceiling limit, apply a GF of say .3 / .8 to a 35 minute BT dive to 80', and you've ended up with a constrained ascent rate of ~10 fpm from 35' fsw ~ 10' fsw, and 3 more minutes of obligated deco at 20 (1 min stop + travel time ) and 10 fsw ( 2min stop + travel ), in addition to a 5 minute stop at 15'. upshoot is your deco ceiling is almost double (-4.9 fsw) that of the straight 30fpm down, 35 at bottom, 30fpm up (-2.44 fsw), but you're spending a lot more time hanging out on a line. the winner for most conservative tissue loading, unsurprisingly, is to trim BT to keep the deco ceiling above 1.0 ATA (or whatever factor you wish to gamble on), at which point you're out of the water with a 7' fsw ceiling to spare; thats with no additional GF factors applied!
also of note, unless you've got gills, you're not gonna hit these situations in a single dive with a single al80. with my best calm rmv on a single al80, rock bottom pushes you into ascent a minute before you pull the deco ceiling below the surface.
Code:
# base NDL checks
$ ./plan --ceil 0 --gf-low 1.0 --gf-high 1.0 --m-value 1.0 --plan plans/ref/naui/test-naui-ndl-080.yaml
diving plan: 80' NAUI NDL test al80, al80
start end fsw in fsw out ps in ps mid ps out time state notes
0 3 0 80 3000 2962 2923 3 +27
3 38 80 80 2923 2232 1541 35 ~
38 40 80 15 1541 1501 1462 2 -32
40 45 15 15 1462 1363 1264 5 sstop
45 46 15 0 1264 1244 1224 1 -15
leading tissues: 18:0.926 27:0.917 38:0.893 54:0.823 12:0.818 77:0.742
0.926 ata, -2.44 fsw leading tap 18.5 ht, 0.853 ata, -4.85 fsw avg leading tap
# applied gradient factors
$ ./plan --ceil 0 --gf-low .3 --gf-high .8 --m-value 1.0 --plan plans/ref/naui/test-naui-ndl-080.yaml
diving plan: 80' NAUI NDL test al80, al80
start end fsw in fsw out ps in ps mid ps out time state notes
0 3 0 80 3000 2962 2923 3 +27
3 38 80 80 2923 2232 1541 35 ~
38 40 80 35 1541 1501 1462 2 -23
40 41 35 25 1462 1442 1422 1 -10
41 42 25 20 1422 1402 1383 1 -5
42 43 20 20 1383 1363 1343 1 stop
43 44 20 15 1343 1323 1303 1 -5
44 49 15 15 1303 1205 1106 5 sstop
49 50 15 10 1106 1086 1066 1 -5
50 52 10 10 1066 1027 987 2 stop
52 53 10 0 987 968 948 1 -10
leading tissues: 38:0.850 27:0.835 54:0.806 18:0.787 77:0.738 109:0.668
0.850 ata, -4.93 fsw leading tap 38.3 ht, 0.781 ata, -7.23 fsw avg leading tap
# with a hard deco limit of 1.0 ATA
$ ./plan --ceil 1.0 --gf-low 1.0 --gf-high 1.0 --m-value 1.0 --plan plans/ref/naui/test-naui-ndl-080.yaml
diving plan: 80' NAUI NDL test al80, al80
start end fsw in fsw out ps in ps mid ps out time state notes
0 3 0 80 3000 2962 2923 3 +27
3 28 80 80 2923 2430 1936 25 ~ deco limit (26 of 35 min planned)
28 30 80 15 1936 1896 1857 2 -32
30 35 15 15 1857 1758 1659 5 sstop
35 36 15 0 1659 1639 1620 1 -15
leading tissues: 18:0.797 27:0.771 38:0.750 12:0.731 54:0.695 77:0.634
0.797 ata, -6.69 fsw leading tap 18.5 ht, 0.730 ata, -8.92 fsw avg leading tap
# single tank rock bottom eviction
$ ./plan --ceil 0 --gf-low .3 --gf-high .8 --m-value 1.0 --plan plans/ref/naui/test-naui-ndl-080.yaml --tanks al80 --rmv .5
diving plan: 80' NAUI NDL test al80
start end fsw in fsw out ps in ps mid ps out time state notes
0 3 0 80 3000 2936 2872 3 +27
3 27 80 80 2872 2082 1292 24 ~ rock bottom (25 of 35 min planned)
27 29 80 30 1292 1226 1160 2 -25
29 30 30 15 1160 1127 1094 1 -15
30 35 15 15 1094 930 765 5 sstop
35 36 15 0 765 732 699 1 -15
leading tissues: 18:0.775 27:0.754 38:0.736 12:0.704 54:0.685 77:0.627
0.775 ata, -7.43 fsw leading tap 18.5 ht, 0.713 ata, -9.46 fsw avg leading tap
so, for the OP's question: a safety stop is a stop taken to increase your safety margin vs a direct ascent, but is not required to provide a deco ceiling above conditions at the surface; a deco stop is a stop required to raise your deco ceiling above conditions at the surface. whats more fun, is that the line between the two can be extremely fuzzy at times, and is somewhat based on the diver's risk aversion level.