If a category is marked as "ON HOLD", all spots are being held by customers currently registering. Some of these spots may become available again, so check back soon. For example, there is only one registration slot left for a category and one person is on the registration process but has not finished the registration yet, then this category is showing as "On hold". Once this person completes the registration, the status will be changed to "Sold out". If this person does not complete registration within 15 minutes registration window, then this category will be open for public to register.
