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The scripting engine behind auto responder email templates and logic steps is Apache Velocity.  It's a Java based scripting engine that runs well within the UltraCart system.

There is a learning curve associated with velocity, but it's also very powerful and fast.

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.

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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