These designations have different meanings for different resources.
Possible values:
low: For CPU: Shares = 500 * number of virtual CPUs For Memory: Shares = 5 * virtual machine memory size in megabytes For Disk: Shares = 500 For Network: Shares = 0.25 * networkResourcePoolHighShareValue
normal: For CPU: Shares = 1000 * number of virtual CPUs For Memory: Shares = 10 * virtual machine memory size in megabytes For Disk: Shares = 1000 For Network: Shares = 0.5 * networkResourcePoolHighShareValue
high: For CPU: Shares = 2000 * number of virtual CPUs For Memory: Shares = 20 * virtual machine memory size in megabytes For Disk: Shares = 2000 For Network: Shares = networkResourcePoolHighShareValue
custom: If you specify custom for the level property, when there is resource contention the Server uses the shares value to determine resource allocation.