This is the documentation for ConfiForms Server/Data Center app
However, this might also work for ConfiForms cloud and in most cases it does. But please see this page to understand the differences between server and cloud versions of the ConfiForms app.
In ConfiForms you can set up the valiations rules to be applied within a context of the whole dataset.
This is done using ConfiForms Field Definition Rules macro and choosing action "Validation rule (dataset)" from the list of actions
The important bits are how to set up the filter when it runs and what to add into the validation rule
The goal
We want to check if a person (by email) has already registered. It is important moment, that we actually want to check by email, and not by built-it operator [UNIQUE], which can easily help us with this if we wanted to check by username (built-in Confluence user object). See more on filters here: ConfiForms Filters
But we want to check by user's email which they type in the registration form. Something like this:
