The remitter need 15CB in all the cases when your aggregate payments along with the present payment exceeds Rs. 500000/-.

Aggregate payment means taking into account all the payment of any kind and payment made to all the companies  togather . The limit is not remittee wise.  E.g. if Company A remitte Rs.450000 to Foreigner for personal payment. After that if the Company A wants to remitt Rs.100000 to another foreign company for Purchase of shares (or for any payment) he needs form no 15CB.