Not all fields are available when selecting to 'Create import file'.
One-to-many fields, such as Constituent Code, are not included because a constituent may have multiple instances of the field (multiple constituent codes) on their record. These fields, which could have multiple field values, each have their own import type (i.e. Constituent Code import, Constituent Address import, Constituent Notepad import, etc.). Therefore, they are not included as options when creating an import file.
These fields must be pulled from the records another way, such as from a Constituent Code import or from export.