What Is a Routing Rule?
A routing rule is a condition evaluated against a submission's contents that decides which destinations receive it. Submissions matching the rule go one way, everything else goes another.
It turns one form into several workflows. A single contact form can send enterprise enquiries to a sales channel, bug reports to an engineering channel, and everything else to a shared inbox, without the visitor choosing a department and without you maintaining three separate forms.
Order and fallbacks are where these get fiddly. If no rule matches and there is no default, a submission can be stored and delivered nowhere, which looks exactly like a bug to whoever was waiting for it. Decide what happens to the unmatched case before you write the first rule.
Rules read the data as submitted, which means they are only as reliable as your field values. Matching on a select with fixed options is dependable. Matching on free text is guesswork with extra steps.
In WebForms
Routing rules are a Pro and Agency feature in WebForms. Without them, every connected destination receives every submission.