This feature is present since version 3.1.0 of the add-on.
To use this new feature, create, or edit an already existing assignee activity. Hard booking, is a standard type activity that was present up to 3.0.6 version of the add-on.
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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.
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):
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Like in case of hard booking's slid background, stripe color is automatically set different for each activity. If an already existing hard-booked activity has been colored 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 green:
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