The Free On-line Dictionary of Computing (30 December 2018):

Packed Encoding Rules

    (PER) ASN.1 encoding rules for
   producing a compact transfer syntax for data structures
   described in ASN.1, defined in 1994.

   PER provides a much more compact encoding then BER.  It
   tries to represents the data units using the minimum number of
   bits.  The compactness requires that the decoder knows the
   complete abstract syntax of the data structure to be
   decoded, however.

   Documents: ITU-T X.691, ISO 8825-2.

   (1998-05-19)