Glossary
UTM tracking & hidden fields
Capturing campaign source data silently alongside form answers.
UTM parameters (utm_source, utm_medium, utm_campaign and friends) are tags appended to a link's URL that identify which campaign sent a visitor. A hidden field on the form reads those parameters from the page and submits them silently alongside the respondent's answers.
This stitches marketing attribution to outcomes: instead of "we got 200 submissions", you can say "the newsletter drove 60 submissions and the paid ad drove 12", and segment completion rate by source to see which channels send people who actually finish.
Hidden fields can also carry context the respondent should not have to type — a referrer id, a plan selected on a previous page, an A/B variant. Because they are not user-entered, validate them server-side too; a hidden field is still client-supplied data and must not be trusted blindly.