This refers to the number of pages of your book interior or manuscript. and also some Custom Trim Sizes like 8.25' x 6', 8.5' x 8.5', etc. There are some Industry Standard Trim Sizes like 5' x 8' (mostly for novels) and 6' x 9' (mostly for tutorial books, textbooks and recipes), etc. You have a lot of options depending on the dimension of your book.
