I believe it announces the fact that they are 1-piecers on the site. The rider and horse are 1-piece and the rider has a choice of separate right arms.
Some of the 1-piecers are very nice. I have the Austrian and Russian Napo colonels sets. They are both sets of 1-piecers and are lovely. I have heard some complaints from other gamers that some of the other 1-piece sets have a skinny, scrunched look to them. The specific allegation was leveled at the Chasseurs-a-Cheval de la Garde.