I would say that “fill out” would mean to fill in all (required) fields in the form and complete it. On the other hand, “fill in” would mean to fill in values in some fields in a form (not necessary complete the form).

I would say that “fill out” would mean to fill in all (required) fields in the form and complete it. On the other hand, “fill in” would mean to fill in values in some fields in a form (not necessary complete the form).

I would say that “fill out” would mean to fill in all (required) fields in the form and complete it. On the other hand, “fill in” would mean to fill in values in some fields in a form (not necessary complete the form).

I would say that “fill out” would mean to fill in all (required) fields in the form and complete it. On the other hand, “fill in” would mean to fill in values in some fields in a form (not necessary complete the form).