Some specifications (e.g., RANAP) contain a production named PLMNidentity. In most of these specifications the production is declared to be of type TBCD-STRING, although this declaration is not correct, as a filler character can appear in the middle of the string, which is not permitted in genuine TBCD strings.

The default behavior of ASN1C is to recognize a production named PLMNidentity (any case) and treat that production as an OCTET STRING, regardless of what the ASN.1 might specify. This special treatment of PLMNidentity can be disabled with the -noPLMN qualifier to the asn1c command. There is also an <isPLMNidentity> configuration file item at production level to declare that a production is a PLMNidentity and will therefore be treated as an OCTET STRING.