Limit specifies an upper bound for CPU and memory resources that can be allocated to a virtual machine.
A virtual datacenter can allocate more than the reservation to a virtual machine, but never allocates more than the limit, even if there are unused resources on the system. The limit is expressed in concrete units (megahertz or megabytes).
CPU and memory resource limits default to unlimited. When the memory limit is unlimited, the amount of memory configured for the virtual machine when it was created becomes its effective limit in most cases.
In most cases, it is not necessary to specify a limit. You might waste idle resources if you specify a limit. The system does not allow a virtual machine to use more resources than the limit, even when the system is underutilized and idle resources are available. Specify a limit only if you have good reasons for doing so.