Stiff icing is used for decorations such as flowers with upright petals, like roses, carnations and sweet peas. Stiff icing also creates your figure piping and stringwork. If icing is not stiff enough, flower petals will droop. If icing cracks when piped out, icing is probably too stiff. Add light corn syrup to icing used for stringwork to give strings greater elasticity so they will not break.
Medium icing is used for decorations such as stars, borders and flowers with flat petals. If the icing is too stiff or too thin, you will not get the uniformity that characterizes these decorations. Medium to thin icing is used for icing your cake. Add water or milk to your icing recipe to achieve the correct consistency.
Thin icing is used for decorations such as printing and writing, vines and leaves. Leaves will be pointier, vines will not break and writing will flow easily if you add 1-2 teaspoons light corn syrup to each cup of icing.