It is never used on a 'c' before vowels, 'e', 'I' or 'y', since these vowels invariably produce a soft 's' sound.
Examples:
● Française or 'French' is pronounced fro-sez, with a soft, nasal 'o'
● Garçon means 'boy', is pronounced gar-so, with a soft, nasal 'o'.