The Collaborative International Dictionary of English v.0.48:

Appel \Ap`pel"\, n. [F., prop., a call. See Appeal, n.]
   (Fencing)
   A tap or stamp of the foot as a warning of intent to attack;
   -- called also attack.
   [Webster 1913 Suppl.]

V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

APPEL
       A P3P Preference Exchange Language (P3P)