The box says they’re acrylic VSTAR markers. They aren’t magnetic nor pins. What are they used for?
Similar markers were used for the GX era of the TCG. Bulbapedia states:
A special GX marker is used to take note of whether the player has already used their GX attack or not.
Source
Because VSTAR Pokemon have a similar mechanic in which their V-Power move can only be used once per game, it's safe to assume these markers serve the same purpose as their GX counterparts.