Tip: Get to know the velocity syntax. It's powerful!
Requirements
For Email Templates: any valid velocity syntax will do, and is totally optional
For Logic Steps: 1) valid velocity syntax and 2) must #set the variable $result to the name of child step.
References
Apache Velocity Homepage: http://velocity.apache.org/
User Guide: http://velocity.apache.org/engine/releases/velocity-1.6.2/user-guide.html
Reference Guide: http://velocity.apache.org/engine/releases/velocity-1.6.2/vtl-reference-guide.html