What is the Density statement in the SVRF file? The Density operation can include arguments such as IGNORE or INSIDE OF. Without seeing the rule, it is hard to say what is happening.
As for how the code works... see the functional description in the Density reference in the SVRF manual: https://supportnet.mentor.com/docs/201403024/docs/htmldocs/mgchelp.htm#context=svrf_ur&id=60&tag=999140 (2014.1 version, but most of the Density command dates back decades. Should be the same for your version.)