In the Koha Manual, for Macros in the Advanced Editor in this section: https://koha-community.org/manual/latest/en/html/cataloging.html#advanced-editor-cataloging All the examples show updating specific subfields, so you have to just guess that you can update the contents of a control field (MARC 00x$) by just leaving out the subfield codes. (Like 000$, 001$... etc.) We should add an example of one of these leader fields.