TYPO3 Core

Basically localizations can happen in two different modes - either the connected mode (localization belongs to a parent language record) or the free mode (localization has a language, but does not depend on any parent).

For fields having l10n_mode set to exclude in the connected mode, the values of the parent language record shall be cloned to all particular and dependent localization records each time the parent record gets updated (or created).
On the other side l10n_mode exclude does not make much sense in the free mode, since there's no concept of any parent in that regard. In this scenario the value is cloned from the parent language record once during the creation of the localization record. In the context of the localization, the field value is shown and can be modified without any restrictions.