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In Drupal 7 and earlier, all imported and edited interface translations are treated equal. The l10n_update module adds some support for tracking customized translations, so when you import new translations from the community, your customizations can be protected. That module does not expose this functionality on the Gettext import or export screens. Now Drupal 8 comes with this tracking functionality built-in and also features on the manual translation import and export screens.
This is mostly an internal book-keeping improvement, there is no API change for people using t(), format_plural() or any other method to translate strings. The new database column added to track this data is in locales_target and is called "customized".
More changes are in the works to build in the rest of the l10n_update module functionality, see http://drupal.org/project/issues/search/drupal?issue_tags=language-ui%2C...