How to build Machining Process ?(Define Geomertical Expression)

Hello

To increase the utilization of 3DEXPERIENCE
I'm thinking about how to build a machining process.
But I want to know one thing.
Method for selecting machining process features
Can you tell me how to configure Define Geometrical Expression?
I tried to figure out how to do this.
Could not find in Documentation or Companion.
Help me.

Thank you.