ITU-T Rec. X.695 | ISO/IEC 8825-6 specifies the rules for applying PER encoding instructions using either type prefixes or an encoding control section.
Encoding instructions are a means of modifying the encodings of ASN.1 types for some specified encoding rule (in this case PER). They can be inserted in an ASN.1 specification in square brackets (much like a tag in the Basic Encoding Rules, (BER)) immediately before the type that they affect (type prefixes), or they can be collected together at the end of an ASN.1 module (an encoding control section).
It also specifies the procedures for developing, registering and publishing new PER encoding instructions from time to time.