CST - Mesh properties - Hexahedral, minimum cell.

Hi, experts:

In CST Studio Suite, the global mesh properties has a setting - minmum cell, I can choose "Fraction of maximum cell near to model". I've searched the meaning of this setting: Minimum allowed cell size is calculated by dividing the maximum cell size near to model by this number. Very short explaination that I still don't understand the meaning.

Does anyone know how to set this value? Thanks a lot~