If your officiating organization uses the Availability Locking feature, they may limit your permission to edit your Availability Calendar once they've created an assignment for you. It is up to your officiating organization (i.e., your scheduler) to decide whether they want to allow you access to change your availability after creating an assignment. Each organization can choose whether or not to turn this feature on or off. You must contact your organization for further information on which decision they've made and why.
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You can read more about the Availability Locking feature in this support article.

If a day is locked, you will not be able to make changes to your calendar on that particular day and will see a message that says, "This day is locked. No further edits can be made."