Using the ASN1C Graphical User Interface (GUI) Wizard - Page 4

Choose Encoding Rules.

Choose from BER, DER,CER, PER, XER or XML by clicking in the option box. Select options for Function Types and Space Optimizations.

These pages have only optional commands for the ASN1C compiler. It is not necessary to select any of these options for the most basic parsing of your ASN.1 file.