cuó is essentially archaic. It survives in two niches. (1) Ritual usage: the Book of Rites prescribes
盐曰碱鹾 'salt is called cuó' in formal sacrificial offerings — so
鹾 was the elevated synonym used in court ceremony. (2) Salt administration: Ming and Qing-era documents refer to
鹾政 (the salt administration),
鹾务 (salt affairs),
鹾商 (salt merchant). Outside these technical historical contexts the character is not used.