Spirent TestCenter uses a "Date based value" system to determine what versions can be used as well as when a license expires. There are 3 date values. They are:
The license rules:
Version Rule - this rule determines what Spirent TestCenter releases can work on a chassis. Note that this doesn't mean it will work due to the Expiration rule.
If the "Application Release Date" <= "Version Date"
then the licenses will work with that version of Spirent TestCenter.
Expiration rule - this rule determines when a license expires.
If the ("Expiration Date" >= today's date) OR ("Expiration Date" = "permanent")
then the licenses will work.
So based on the date values in the above screen captures:
Version Rule: This chassis can use any version of Spirent TestCenter released on or before December 2011. This happens to be STC 3.9x and below.
Expiration Rule: After April 7, 2012, this chassis will not be able to run any tests. Note that most customer will get a permanent license and so the Expiration rule typically doesn't apply.