Expandable reservation parameter
The Expandable reservation parameter can be used to allow a resource pool that does not have the required resources to request resources from its parent or ancestors. The search for resources goes through the ancestry of the root resource pool or to the first resource pool that does not have the Expandable reservation option turned on.
Here is an example (image source: VMware):
Note that all resource pools have the Expandable reservation option enabled. What happens if we power on all virtual machines in the eCommerce Apps pool? Because the total amount of VM CPU reservation (500+500+500+500=2000) in this pool is greater than the amount of CPU reserved for the pool (1200), the remaining 800 Mhz will be taken from the Retail resource pool, who has 800 MHz to give. If the Retail pool had no more reservation to give, the amount of CPU needed would be taken from its parent, the root resource pool.