If the SEQUENCE type contains an open extension field (i.e., a … at the end of the specification or a …, … in the middle), a special element will be inserted to capture encoded extension elements for inclusion in the final encoded message. This element will be of type ASN1OpenExt and have the name extElem1. This field will contain the complete encoding of any extension elements that may have been present in a message when it is decoded. On subsequent encode of the type, the extension fields will be copied into the new message.

If the SEQUENCE type contains an extension marker and extension elements, then the open extension type field will not be added. Instead, the actual extension elements will be present. These elements will be treated as optional elements whether they were declared that way or not. The reason is because a version 1 message could be received that does not contain the elements.