From Vassal

A free-floating window in VASSAL that lacks a menu or toolbar. Palettes hold cards, units, weapons, and markers. Players can drag these from a palette to the map.

Many palettes have specific spaces for specific types of cards, units, weapons, or markers.

Moving a Piece to a Palette

Most pieces are moved to palettes automatically by right-clicking. For example, right-clicking a unit and then clicking Eliminate sends the unit to the first open space of its side's casualty track on the Dead palette.