The IMPORT_ONCE modifier tells Interchange not to re-import the database from the ASCII file every time it changes. Normally, Interchange does a comparison of the database file modification time with the ASCII source every time it is accessed, and if the ASCII source is newer it will re-import the file.
IMPORT_ONCE tells it only to import on a server restart or catalog reconfiguration:
Database products products.txt TAB Database products IMPORT_ONCE 1
SQL databases don't normally need this. They will only be imported once in normal operation. Also see NoImport for a way to guarantee that the table will never be imported.
IMPORT_ONCE is always in effect for MEMORY databases. A catalog reconfiguration is required to force a change.