While some new licenses have ben introduced for DP 7 and 8, the rest of the license structure is intact
This message
"On-line Extension for ONE Windows / Linux system"
indicates that you have an Application component installed on at least one of the servers in your cell, like Exchange, Oracle, SQL, something like that
For each server where you have this component installed, you need an on-line extension license
Data Protector reads from the top of the Cell_info file, and allocated on-line extension licenses until they are gone, then starts issuing this warning
You don't specify the OS-type and version of your cell manager, so I can't tell you were to find the cell_info file, but, you will need to look at it, and decide if you really need those application components installed. SOme folks will install components "Because they are there", without considering the licensing ramifications