When is it correct to use Attn: (attention) and when is it correct to use C/o (in care of)?
1. Attn: Mr Carey
2. Mr Carey c/o Mr Michael
If #1 appears in an address (e.g. on a letter), it means "This letter is for the attention of Mr Carey".
If #2 appears in an address, it means "This letter is for Mr Carey, care of Mr Michael".
"Care of Mr Michael" means that Mr Michael is keeping Mr Carey's mail for him.
Not a professional ESL teacher.