The term "margins" is widely used to describe the area on a page that is reserved and unable to be printed. The term "fields" is widely used to describe fields within a form and is not widely used in American English to mean the unprintable area at the edges of a paper. Google searches on the two terms reinforce the idea that MARGINS is the appropriate word for the unprintable area at the edges of a page. Many word processor programs use the term margins.
Please change the Layout Mode screen in Valentina and replace the word "field" with the word "margins". This will be make the intent of the selection easier to understand.