In the Nactation tutorial, the "convenience clause" is introduced at diagram #6.
It makes sense to learn to nactate the opening moves first. The purpose of the convenience clause is to make Nactation more user-friendly, as "S" is the first letter in Split. The only standard opening move for which Z is needed is with a roll of 43. By looking at diagrams #5 and #6, the reader can easily compare the difference between White's plays of S (Split, 24/20 13/10) and Z (reverse split, 24/21 13/9).
Some people would prefer using Z for all the small-number splits -- even for 51 played 24/23 13/8, where both 24/19 and 13/12 are blocked -- out of a heightened sense of consistency. In part because the potential confusion that would occur (e.g., file-naming bifurcation), the rest of us can be grateful that they don't insist on their right to never using the convenience clause.
Some people use the convenience clause more than I do. I use it for any position where the small-number split "looks" like the S play in backgammon's opening position (even if it occurs in the middle game). If it doesn't, I use Z.
         
 
         
 | | White is Player 2
score: 0
pip: 164 | 2 point match
Crawford | pip: 167
score: 1
Blue is Player 1 |
|
| XGID=-aa---E-C---dEa--c-e----B-:0:0:1:53:1:0:1:2:10 |
| 21S-53 |
| 1. | XG Roller++ | 24/21 13/8 | eq: -0.3178 |
| Player:
Opponent: | 48.78% (G:13.49% B:0.65%)
51.22% (G:14.67% B:0.52%) | |
|
For 21S-53 at gammon save, S (Split) is "tied" with P (Point).
On the opening move, 53S (Split, 24/21 13/8) is second best but definitely a mistake. I have never seen an author (even centuries ago) recommend it, and I know of only one expert who regularly played it (about four decades ago). For this reason, even though I prefer S, I consider using Z for opening 24/21 13/8 to be less problematic than using Z for the parallel opening splits with 51, 52 and 54.
In short... As with the opening position, for the position above, I much prefer using S for the splitting play with 51, 52 or 54. For the splitting play with 53, my preference for S over Z is not as strong.
  
| | White is Player 2
score: 0
pip: 159 | Unlimited Game
Jacoby Beaver | pip: 160
score: 0
Blue is Player 1 |
|
| XGID=-a----EBBa--eD---c-e----B-:0:0:1:53:0:0:3:0:10 |
| 61P-62S-53 |
| 1. | XG Roller++ | 24/21 13/8 | eq: +0.1257 |
| Player:
Opponent: | 52.23% (G:14.23% B:0.56%)
47.77% (G:10.09% B:0.38%) | |
|
Above is the third roll of the sequence used to reach Stick's variant 1, except I've inverted the situation so that (instead of White) Blue is on roll. Again, with 53 played 24/21 13/8, my preference is S.
Below are examples where I use "Z" even though the convenience clause allows the option of "S."
 
| | White is Player 2
score: 0
pip: 161 | Unlimited Game
Jacoby Beaver | pip: 167
score: 0
Blue is Player 1 |
|
| XGID=-b----E-C---eE---b-d-b--B-:0:0:1:32:0:0:3:0:10 |
| 42P-32 |
| 1. | XG Roller++ | 24/22 13/10 | eq: -0.2984 |
| Player:
Opponent: | 43.48% (G:10.74% B:0.50%)
56.52% (G:18.60% B:0.85%) | |
|
 
| | White is Player 2
score: 0
pip: 162 | Unlimited Game
Jacoby Beaver | pip: 156
score: 0
Blue is Player 1 |
|
| XGID=-a--a-E-C---dFa--c-e----A-:0:0:1:52:0:0:3:0:10 |
| 65R-32S-52 |
| 1. | XG Roller++ | 24/22 13/8 | eq: +0.0827 |
| Player:
Opponent: | 52.62% (G:12.34% B:0.60%)
47.38% (G:12.94% B:0.38%) | |
|
 
| | White is Player 2
score: 0
pip: 155 | Unlimited Game
Jacoby Beaver | pip: 168
score: 0
Blue is Player 1 |
|
| XGID=-b----E-C---cE-b-c-c--b-AA:0:0:1:62:0:0:3:0:10 |
| 63R-33N-62 |
| 1. | XG Roller++ | Bar/23 13/7 | eq: -0.5253 |
| Player:
Opponent: | 38.61% (G:9.19% B:0.36%)
61.39% (G:21.54% B:1.22%) | |
|
 
| | White is Player 2
score: 0
pip: 194 | Unlimited Game
Jacoby Beaver | pip: 194
score: 0
Blue is Player 1 |
|
| XGID=-bb---D-C---dD---c-d---BB-:0:0:1:51:0:0:3:0:10 |
| Nackgammon opening 51 |
| 1. | XG Roller++ | 23/22 13/8 | eq: -0.0084 |
| Player:
Opponent: | 49.70% (G:13.51% B:0.64%)
50.30% (G:13.76% B:0.75%) | |
|