Importing custom field values rely on the exact same XLSX format as provided by the export, so you may use an exported file directly as the source for import.
Doing partial updates
Except for the required A and B columns, you may remove any columns from C and forward as you see fit, but, if present, the header value must match a defined field name for the type.
Same applies to rows - only objects directly referenced by an _id value is updated, making it possible to do fast, partial updates. Shop or Global level imports don't use the _id field but will always update the provided custom fields for the shop being imported into.
Imports run in the background. You can safely leave the page while the import runs, but you are encouraged not to make any changes to the custom fields while the import runs. If you start additional import jobs, the data will be added to the import queue and imported in the order of upload