I believe their are 4 different use cases. The first is the URL parameter. This is when you put in certain values into a url via the url builder, and the form sucks those values into the hidden fields to give you things like the source, or lead source detail, or the asset type, and asset type detail.
The refferer can refer to the referring url. This is just where they previously came from.
There are also referrer's used in a referral program. Use cases for this can be found here: https://community.marketo.com/MarketoArticle?id=kA050000000LAneCAG
Referrer parameters refer to where the person was previously and you can pull data into the form there too. The URL paramater is purely about the current page, and the referring paramater is about the previous page. Does that make sense?
Here is an article that might help: https://community.marketo.com/MarketoArticle?id=kA050000000LH7uCAG#url-parameter
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