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First, you will run Step Hooks before matching. This will let you see what fields the user has mapped, and any fields you want to include that weren’t in the file can now be marked for creation.
Now that you have detected which fields are not present, you can use `addVirtualField` for the missing fields.
This process will allow you to include validation to check for headers of nullable columns--essentially, you can QA the headers in your data even if nothing is contained within those headers.