Priorities are hints that can optimize cache scavenging. Essentially, if the ASP.NET engine decides to remove cache items because memory is scarce, it will remove items with a lower priority cost first. The CacheItemPriority enumeration also includes a NotRemovable member that you can use to prevent ASP.NET from removing an object from the cache automatically when memory is low.
public enum CacheItemPriority { Low = 1, BelowNormal = 2, Normal = 3, Default = 3, AboveNormal = 4, High = 5, NotRemovable = 6 }