The PVU license is used for servers. The license is based on the capacity of
computing capacity of the computer. This capacity is expressed in PVU's ( processor value unit ).
A larger capacity results in a more expensive license. Increasing cores will need a new
additional license to cover the new capacity.
More on PVU
This page will give some pointers about PVU licensing. This licensing mechanism is applicable to various server
components in Rational Performance Test Server and Rational Test Virtualization Server.
Where the licensing with the VU is "each simulated user needs a VU". So with 1000 VU's you could simulate 1000 concurrent users.
PVU price is based on the computer it runs on. The capacity of the computer will limit the number of concurrent users which can be simulated.
A low interacting test setup might be able to run 10.000 users from a box. That same box might have the capacity to simulated 1000 highly interacting users.
Furthermore for the performance testing of web-services you are more interested in transactions/sec instead of simulated users. You ask for capacity.