An optional process calls an entire Flow Rule. This could be something like starting an investigation in an insurance claim.
An optional action is a Flow action and generally performs a single purpose. An example of this is sending an email.
You can add both in a similair in the Stage Designer, however they are presneted slighly different to the end user. They are both found under Other Actions; the Action is lsited directly, The Flow is found under Other Work.