Help me on wedding gowns! What is off-the-rack, sample & brand new gowns

I've been getting confusing advise from friends, can someone please help. What exactly is off-the-rack gown? What is the difference between off-the-rack and brand new gown? Is off-the-rack gown considered a BRAND NEW gown? (some shops said so) What to expect from an off-the-shelf wedding gown? Will it require fixing? How about brand new gown? Will they be in a better condition without defects? I want a lace gown, so I am worried about defects on off-the-rack gown given its delicate nature.