I assume this is for the armholes. I don't know how your pattern is written but I'll take a stab at explaining. When you do the right side the armhole is on the right and the left side armhole is on the left.

This means if the pattern says in reverse it means the decreases are on the opposite side to what they were on the first half

If you started with even stitches and decreased at the end on the first half then start with the decrease and end even on the second piece.

I hope you can understand what I mean

This pattern is from the vintage Gypsy Vest pattern book.
Row 1-Work 4 dc in 3rd ch from hook *skip 3 ch.,1 dc, ch 1, 1 dc in next ch (cluster made), skip3 ch., 7 dc in next ch. (shell made) repeat from * end skip 3 ch. 4 dc in next ch., ch 3 turn
Row 2- 1 dc in each of next 3 dc *skip 1 dc, cluster, in next cluster, skip 1 dc, 1 dc in each of next 5 dc; repeat from *end last. Repeat, skip 1 dc, 1dc in last of the next 4 dc., ch 3, turn
Row 3-1 dc in first dc. * skip 2 dc shell in next cluster, skip 2 dc cluster in next dc. repeat from * end last repeat skip 2 dc., cluster in next dc. ch 3, turn
Row 4-1 dc in cluster *skip 1 dc., 1 dc in each of the next 5 dc., skip 1 dc cluster in next cluster, repeat from *end last repeat cluster in next dc ch 3, turn
Row 5- 4 dc in cluster * skip 2 dc cluster in next dc. skip 2 dc., shell in next cluster repeat from *end last repeat 4 dc in next dc ch 3, turn
Repeat from Row 2 for pat. st. Work even until piece measures 14 inches or desired kength-9 pat. rows. End in the 4th row.
Left Front (small) Wprk 1 cluster at beg (3 full shells), end with a cluster. Next row at front edge only leave off cluster every other row until 1 full pat and 2 clusters and a half pat remain. On all sizes work even until armhole measures 8 inches. Finish off. Work other front to correspond.
To pick up back leave a full pat and cluster for armhole each side. Start half pat each end keeping each end until armhole measures same kength as fronts.
Do you understand any of this pattern? I'm totally confused. If you can understand the pattern and can help me with this vest, I'd sure appreciate it. Thank you in advance. Margaret