Here's how it typically works:
PPRuNe, for example, decides to be a Viglink affiliate. It installs a bit of gubbins on its web site.
Someone posts a message saying 'I bought my rubber duck at "x", and it was a very nice rubber duck.' If retailer "x" is one of Viglink's 30k clients, then the link is replaced by one which goes through Viglink, and PPRuNe gets a referral fee if a sale is made through that link.
In this case, PPRuNe is IB so I assume IB Is using Viglink across many of its sites. Viglink claims to be the #2 monetization tool after Google Ads.