The "Proc-Type:" encapsulated header field, required for all PEM messages, identifies the type of processing performed on the transmitted message. Only one "Proc-Type:" field occurs in a message; the "Proc-Type:" field must be the first encapsulated header field in the message.
The "Proc-Type:" field has two subfields, separated by a comma. The first subfield is a decimal number which is used to distinguish among incompatible encapsulated header field interpretations which may arise as changes are made to this standard. Messages processed according to this RFC will carry the subfield value "4" to distinguish them from messages processed in accordance with prior PEM RFCs. The second subfield assumes one of a set of string values, defined in the following subsections.