This is a list of keywords, which is used in Contribution Link of miniBB. Is using only a certain, constant keywords phrase for each certain domain. In the most recent versions of miniBB, the link appears in the bottom right corner of each forum, and it's a miniBB default icon/logo. When you mouse over it, as the ALT or TITLE tag, one of such keywords should appear. For example, here on miniBB forums, the link says "miniBB Support Forums Powered by Forum Software miniBB
In other words, the link is unique for each domain, and it's built upon the formula: Forums Title + Keywords Phrase + miniBB. That's done for crazy SEO fans, taking care about the validity of 3rd party links.
It's just the price of using a free software. The only legal way of going around of it, is to purchase a Commercial License
, which allows to remove the miniBB attribution link.