Exporting a tab delimited text file from a record contains also the info carried over from a related file through a lookup field. Information in a lookup field is static. Once copied over from the parent database, the information inside this lookupfield stays unchanged, even if the content of the parent field in the parent databse is changed in the meantime. Existing contents will get overwritten. Lookup fields are static.
Lookups present static information.
With a tax based on a lookup field, the information in the older invoices remains unchanged, and therefore correct. Only newly created invoices will be based on the new tax level. In a relationship an exact match is an absolute necessity in order to obtain related records. The dialog box of the lookup definition contains 4 radio buttons, enabling the user to define an exact match, copy an item lower or higher than, or put up a dialog box with a message. FMP's solution is not to support them directly.