Hi Jimbo,
Blue1978 is correct that the REM Management Console allows you to do delta and trending analysis on Retina scans. I'll give an example...
Scanner 1 scans Network A once a month
Scanner 2 scans Network B once a month
In REM, I can run a vulnerability report of the results for the same month to combine the results of Scanner 1 and 2.
In REM, I can run a vulnerability delta report for last month and compare to this month to see whats new, removed, and unchanged. The delta report can also be used to compare 2 scan jobs too (ie instead of 2 date ranges).
Hope that helps.