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, is a standard type activity that was was the only possible one present up to 3.0.6 version of the add-on..png?version=1&modificationDate=1538380674336&cacheVersion=1&api=v2)
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Since 3.1.0 version of the add-on, when following the typical way to create, or edit one gets a new feature "Soft-Booking" (1) present.
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After getting it ticked, instead getting a standard hard booked activity, marked with full color filling (2), you ought to see diagonal stripes and white background (1):%20-%20comparison%20of%20booking%20types.png?version=1&modificationDate=1538381670822&cacheVersion=1&api=v2)
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Like in case of hard booking's slid solid background, stripe color is automatically set different for each activity. If an already existing hard-booked activity has been colored associated with i.e. green color then if you create a soft booked activity of the same type, color of the stripes is to be also set green:%20-%20new%20soft%20booking%20activity%20placed%20on%20the%20graph.png?version=1&modificationDate=1538382294979&cacheVersion=1&api=v2&width=1083)
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