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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.
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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:
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