The typical bar positoin is shown below by Technical Rules Book
Does there exist any rule that forbids the bar to be as low as possible behind the back?
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This is governed by the first rule:
"1. The lifter shall face the front of the platform. The bar shall be held horizontally across the shoulders, hands and fingers gripping the bar. The hands may be positioned anywhere on the bar inside and or in contact with the inner collars."
where inner collars refer to the 810 marks as shown here or page 5 of the Technical Rules here. So short-armed person can have the bar as low as possible but the bar must touch the shoulders. By placing the bar as low as possible, the lifter gets less torgue on the spine.
Real examples about the positions can be seen here.