How can I create a rate only visible to the staff?

To have a rate only visible for the staff and not the guests, you must create a Private rate linked to a voucher.

Note: 

• Private rates are included in both Mews Core and Mews Pro.
• Mews offers two subscription packages, Mews Core and Mews Pro. To see which features each package includes, see Understanding the Mews Core subscription package and Understanding the Mews Pro subscription package.