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.
Smart multi-row is a new addition to ConfiForms field types and "smart fields" family in particular. This field type enables you to create master-details forms much easier than before.
It has been introduced as beta in 2.21.1 - Release Notes#Version2.21.1 and as GA in version 2.24.7 - Release Notes#Version2.24.7