You will find that there are scores of configuration settings that are not documented and only told to you by Netezza if the default settings of the appliance are not optimal for you.  factrel_size_threshold is one of those configuration settings.

Basically, it works just as you described. What additional information are you looking for?  Several of these configuration settings are specific points that impact a single piece of functionality in the system.  Or were you looking for additional settings that could be tweaked?

We have a few different settings, depending on the applications we run on a given NPS.  What we did to evaluate the optimal setting, we checked to see what the largets dimension size was in all of the databases on a particular serve.  Then we looked to see which facts fall below that value and how far above that value of ther facts were.  For one server with similar databases, we have 2 or 3 fact tables in our larger databases that we never want broadcast.  We made sure that the threshold was below those row counts.  On the same server, we have a couple of dimensions that are very large, but we still would rather have them broadcast than the (larger) fact that they are typically joined to.  So, we made sure that the threshold was above that row count by enough to accommodate for growth over the next few years.

We did not need to set the value until we saw queries suffer because the fact dataset was being broadcast instead of the dimension.  As validation, we saw the plans for these queries change to what we expected.