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Sounds like the rule file has a DRC rule, OD.DN.3, that is turning up some violations of a PDK rule. From the rest of the headline, it has something to do with density on "All_OD" (is that a layer?) in a 20 micron by 20 micron region. The rule has the regions being stepped a half-region each time.
As for what the rule means... that depends on your design. The PDK should have more information. What does the RDB show? (You should be able to highlight errors in your layout with Calibre RVE.) You might want to run through some of the training material -- reviewing the errors from a DRC run is part of the basics.
Hopefully you solved this already, though; looks like the forums have been pretty quiet this week.