In Straight Razor Place Wiki there is a text about 'Bevel-setting in theory and practice'. The website link is Bevel-setting in theory and practice - Straight Razor Place Wiki

I do not understand the sentence 'But in our real world practice, we use flat abrasive tools, and those tools are only effective with flat bevel sides that lay in the same field as the imaginary field that touches the spine and the edge'.

Can someone please illustrate this for me with a sketch?

Thanks in advance.