Why custom events exist: When you add the UET tag tracking code to your website, it makes a call to Bing Ads every time a user visits that page. You can then define conversion goals to record visits to particular webpages (thankyou.html for example) as conversions. However, you might have webpages that drive more than one type of conversion. For example, you may allow people to both subscribe to a newsletter and download a white paper on the same webpage. In such case, it is insufficient to just know that someone visited that webpage if the goal is to track the subscriptions and white paper downloads as separate conversions. That is where custom events come in. Another example would be that you have "click out" from your site to other seller's pages or websites that you refer traffic to. Since you don't have permissions to implement tags on those sites, you can track how many times you send a customer to another site via custom events.
Now, let's look at the two steps:
Step 1: Reporting custom events using UET: There are 4 parameters that advertisers can report when logging custom events.
To learn more, see How to report custom events with UET
Step 2: Creating custom event type goals: When creating custom event type goals, you must specify the values for one or more of the parameters. Bing Ads will match these values with the custom events being logged via UET to count them as conversions.
Some examples below: