Сзади describes relative spatial position. Позади often (and normally) refers to something previous in logical or/and time sequence. If you left something behind forever it is позади. Sometimes позади also used in spatial sense (and in that case is interchengeable with сзади). But позади in that case describes only a temporary position of the object that is if you turn around it will not be позади you any more. If the spatial position is constant (that is attached to your behind like backpack) it is only сзади.
Total:
позади - previous in sequence or temporary spatial position
сзади - all kind of spatial position
More interesting is the difference between "вверху" and "наверху".![]()