In most cases, users in your organization will set themselves as available for all the tasks they can work on. But occasionally your users may need to set themselves as available only for certain tasks at certain times. For example, maybe someone on your team usually works on Chat support, but helps with Phone support one day a week.
For these situations users can set themselves as partially available. In My Schedule click any available time and adjust the checked tasks in the popover to just the tasks you can be scheduled for in that time:
Planner will also show a striped background for partially-available time. If you’ve selected a task that the user is available for the stripes will be gray. If you’ve selected a task they are not available for the stripes will be red. Hover your mouse over the striped background to see which tasks are available.