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.
With recent update to ConfiForms (from version 1.27+) you can enable the rules to filter out choices on smart and db fields based on the values in another field (or fields)