Sentence:  However , the possible absence of a center of symmetry not only moves the hydrogen atom off **f , but also allows the oxygen atoms to become nonequivalent , with **f at **f and **f at **f ( space <WSD>group</WSD> **f ) , where **f represents the oxygens on one side of the **f layers and **f those on the other side . 
Meaning of the word marked with <WSD> tags:  any number of entities (members) considered as a unit