Does SecureDNS enabled means Avast DNS servers are used for internet & not systems DNS servers?
Yes 
It is programmed so that it is used, and not your isp or other dns services.
Greetz, Red.
I think during install this should be mentioned that SecureDNS enabled will use Avast DNS instead of ISP DNS or system DNS.
SecureDNS enabled gives randomly “check if the site name entered is correct”.
SecureDNS disabled gives no probs.
And this is confirmed. I am checking for past few days.
And does SecureDNS has malware/phishing protection?