Approval rule configuration using 'Enter a function' validation

Summary

While configuring DOR approval, we wanted to lookup the document code from table values and return a value using the function

Content

While configuring DOR approval, we wanted to lookup the document code from table values and return a value using the function.

Our question:

1. How this 'Enter a function' works in approval rules

2. In which programming language this function needs to be written

3. How this will be invoked

Attaching the screenshot of this function from approval screen

Regards

Ranjit