They aren't really reaction commands. Remember in KH1 how things like opening chests, examining points of interest, (talking to people?,) using special abilities like Strike Raid and so on were in the bottom item on the mini menu? It shared the slot with summons, so you had to scroll horizontally between the version of the menu that had the extra commands and the version that had Summons. They nixed that and everything that was in the bottom slot (not Summons) is now mapped to the triangle button like it has been in every game since KH2.
So they didn't add special reaction commands to boss fights or anything, that's just the marker to indicate that a special move like Ars Arcanum can be used or else it's just a field action.