This feature is present since version 3.1.0 of the add-on and allows to book activity assigned to a particular user, or resource to which you cannot fully be sure about (e.g. user who doesn't have a valid job permit issued yet).
To use this new feature, create, or edit an already existing assignee activity. Hard booking, was the only possible one present up to 3.0.6 version of the add-on.
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