Hi Iris
I can't help you with what you are seeing within Moz without seeing it specifically, but maybe some from support will be able to help you with that.
The ?db at the end of the web address for your site is called a query string. Typically, these are used to pass a string of text (in this case 'db') to run against a script on the site to do something as a result of receiving that string of characters. But it may be that it doesn't do anything and if, on the web server, there us nothing to process the query string, then it will be ignored. For example, if you try this http://www.bbc.co.uk/?db and this http://www.bbc.co.uk will do the same thing because there is nothing to process the ?db on the receiving web server.
You can only ask the person who created the link with the ?db to give the reason why.
I hope that helps in some way,
Peter