This is Register control. It's used to register user phone number into reminder group for remind back to the caller later.
Before add this control to your call flow, you have to create the reminder group first. Click here to learn more about how to create reminder group.
Click on Register control to add to your call step and setting up as the below:

• Confirmation: is where you can explain to tell to the caller that caller's phone number will be register to system for reminder back next time. Click on setting button ( ) next to the confirmation label and set the explanation.
• Reminder group: click on drop down list and choose the reminder group for storing caller's phone number.

Click button Save to finish.

This screenshot showing that the caller's phone number will be registered to reminder group Pregnancy follow up.