There is no setting in Attaché that allows the number of decimals displayed to change by individual product in any form.
The settings for a form are controlled by line type. So, if three (3) decimals are set for Decimals in Quantity in Setups, Products, Options, Product Options, then up to 3 decimals could be displayed for all Product Lines in any one form.
A separate form could display one decimal, but all product lines on that form would do the same as the Decimals setting in the form field would be formatting the displayed value.