How to understand the "sets" of this "$system.OBJ.GetQualifiers" method?

Guessing a typo(copy paste from SetQualifiers?) in the documentation. It should say "...otherwise it gets the default qualifiers for this namespace."

@Cryze Zhang - it looks like this was an issue in prior versions as well.  

Did you know you can report this directly to the InterSystems Documentation team?  Click on the blue "Feedback" button on the right side of the screen and it will give you a chance to tell Docs directly what is wrong with the page.  Here is the 2021.1 link you can use for your convenience:
https://docs.intersystems.com/irislatest/csp/documatic/%25CSP.Documatic…
 

Thank you for noticing this and bringing it to our attention!  Please confirm once you have reported the issue and it will get addressed.