Importing:

If you set an “overwrite value” in the settings, and the import contains the set “overwrite value”, the imported issues will have no value in them (custom fields with default values set will be imported empty)
If you leave the column blank during the import, the imported issues will have the default set value in them (custom fields with default values set will be imported with the default value filled in)

Exporting:

By default (the highlighted checkbox is not checked)
- If the issue doesn’t have value in the custom field, it will come out empty in the export
- If the issue does have value in the field, it will come out with the value in the export
Checked (the highlighted checkbox is checked)
- If the issue doesn’t have value in the custom field, it will come out with the "overwrite value" in the export, which makes it easy to re-import
- If the issue does have value in the field, it will come out with the value in the export

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