Resource Management Guide : Understanding and Managing Resource Pools : What Are Resource Pools?

What Are Resource Pools?
Use resource pools to hierarchically partition available CPU and memory resources.
Each standalone host and each DRS cluster has an (invisible) root resource pool that groups the resources of that host or cluster. The root resource pool is not displayed because the resources of the host (or cluster) and the root resource pool are always the same.
 
If you do not create child resource pools, only the root resource pools exist.
Users can create child resource pools of the root resource pool or of any user-created child resource pool. Each child resource pool owns some of the parent’s resources and can, in turn, have a hierarchy of child resource pools to represent successively smaller units of computational capability.
A resource pool can contain child resource pools, virtual machines, or both. You can create a hierarchy of shared resources. The resource pools at a higher level are called parent resource pools. Resource pools and virtual machines that are at the same level are called siblings. The cluster itself represents the root resource pool.
Figure 3-1. Parents, Children, and Siblings in Resource Pool Hierarchy
In Figure 3-1, RP-QA is the parent resource pool for RP-QA-UI. RP-Marketing and RP-QA are siblings. The three virtual machines immediately below RP-Marketing are also siblings.
For each resource pool, specify reservation, limit, shares, and whether the reservation should be expandable. The resource pool resources are then available to child resource pools and virtual machines.