Said of a charge which is conjoined to another charge to base, e.g. a falcon perched atop a gauntleted cubit arm fesswise is in pale a falcon conjoined at the feet to a gauntleted cubit arm fesswise. See also Above, Upon.
Precedent from the Early Days:
ATOP Atop means the [charge] is standing on top of the mount. Upon means the [charge] is charged on the mount. WVS  [LoAR 13 Aug 81], p. 1 http://heraldry.sca.org/precedents/wilhelm/atoc.html#atop