the symbol for principal focus and focal length is f. so how do we differentiate it?

Principal focus is a point (on the principal axis) where parallel incident rays meet after reflecting (in mirrors) or refracting (in lenses). It is a fixed point of particular lens/mirror and depends upon its shape. Principal Focus also determines the focal length of the lens/mirror. So although the letters are same but they have different physical significance focal length is an theoretical term which represents a quantity while principal focus is a diagrammatic term basically