I was afraid you would ask that!
Thank goodness and the internet for this list!  Found it in the archives.
Here is the text that was in the "new stuff" part of the server 4.1 README.

(And it isn't necessarily "backed up" in 30 days; I think it is any type of
session.)

********************************************************************
$$1 License counting changes for "in use" *
********************************************************************
With this service level the following changes to in use license counting are
introduced.
- License Expiration. A license feature that has not been used for more than
30 day will be expired from the in use license count. This will not change
the registered licenses, only the count of the in use licenses. Libraries in
use will not be expired, only client license features.
 - License actuals update. The number of licenses in use will now be updated
when the client session ends. An audit license is no longer required for the
number of in use licenses to get updated.


-----Original Message-----
From: David E Ehresman [mailto:deehre01 AT LOUISVILLE DOT EDU]
Sent: Monday, May 19, 2003 3:31 PM
To: ADSM-L AT VM.MARIST DOT EDU
Subject: Re: Old Client Filespaces / Licenses


>Well, that's the way it was implemented starting at 4.1, and was
>documented
>to work that way.
>
Where is/was it documented that a node not backed up in 30 days is not
counted in the license in use count?

David