What happens if the query parameter isn't on the link?

The click redirects to the fallback destination. Every Redirect by Query Parameter link has exactly one fallback — it's used whenever the incoming URL doesn't carry any of the parameter/value pairs you've mapped, when the parameter is present but its value doesn't match a destination, or when the parameter is empty.

Set the fallback to the safest or most common destination — usually your main landing page — so visitors never hit a dead end when the link is shared stripped of its parameters (by a messaging app, a URL cleaner, or a user copy-pasting the bare URL).