How would you choose between two characteristics to be used for developing a hierarchy in classification?

Dear Student

Gross character will form the basis of start of the hierarchy and fine character will form the basis of further steps of single the hierarchy.
Example:
Presence of vertebral column in human beings can be taken under vertebrat.
Presence of four limbs makes them members of Tetrapoda.
Presence of mammary glands keeps them under mammalia.

Thanks