@Davide thanks, i have to find another solution then,
plus the custom fields - in line occupy a lot of space since their column seems to have a fixed width, even if i put only 2 letters
Maybe your solution is to create the custom field in Non-inventory items (and inventory items). It will probably require creating many Non-inventory items but you get all these benefits:
The details appear on each line in documents (view mode)
Nothing appears in the line if custom field is not applicable - so, doesn’t take up space (view mode).
No extra line fields in edit mode to take up space
Can be selected in custom reports (line item custom fields are not available in custom reports)
Drop down list is available
One custom field covers many documents (SI, PI, DN, CN, etc)