GNSS Horizontal Integrity Limit/ Horizontal Protection Level is a measure of GNSS position error/integrity that's very important in RNP operations and RAIM, but it has an obtuse definition in aviation regulations like DO-229D that's hard to apply in practice:
[HPL] describes the region assured to contain the indicated horizontal position. It is a horizontal region where the missed alert and false alert requirements are met for the chosen set of satellites when autonomous fault detection is used.
Is there a simpler definition that is equivalent for most equipment? It seems like a 1 - 10-X% accuracy bound, given pessimistic assumptions about satellite availability, but given the description it's hard to be sure?