The Advanced Functions and Numeric Buttons section is where a lot of the function buttons you will need throughout the course of a sale to speed the process up are located. Regardless of what Tab you select from the bottom of the screen, these buttons will remain constant on the POS.
This section is totally customisable, but it should be noted that you change this section sparingly. There is a limited amount of space in this section and a need for function should outweigh the need to feature. The numeral buttons should remain on the screen somewhere central as default, as you will use these many times over the course of various sales.