Because our main developer started as dedicated PHP developer in his journey with VtigerCRM, which complain about not having any chance to modify existing functions, he take a look to implement lot’s of interfaces into our modules.

If you are a PHP developer you could extend the Workflow Designer on lot’s of interfaces.
I implement lot’s of these interfaces only for the 6.x version. If you are still using 5.4, you probably don’t found the directories to create files.

The most important fact for your should be: Don’t modify any files originally created by the core Workflow Designer.
This files will be overwritten with every update and your modifications are lost.

All interfaces are stored in the directory extends of Workflow Designer.

I started to exclude lot’s of functions also into such “Extensions” and not implement everything into the core.
Because everything within the extends directory is unencrypted you have the chance, to modify them by your needs.