The "Check Availability" function during media plan creation is designed to check the ad space and capacity availability in the ad server (D). This process ensures that the desired placements and inventory are available for the planned campaign dates and volumes, thereby avoiding overbooking and enabling effective campaign planning and execution.References:
https://help.salesforce.com/articleView?id=sf.dmp_advertising_sales_check_availability.htm
https://www.salesforce.com/products/media-cloud/overview/