In R12 suppliers are represented in the Trading Community Architecture (TCA). This way you have access to the features provided by TCA for the suppliers.

A party record is created automatically when you create a supplier in AP. Party records are created in HZ_PARTIES table. That party is created for the supplier without any account information.

That is why suppliers set up in Payables could be viewed in Receivables even though they have no account information. The search feature in Receivables gives you both parties and accounts. Since suppliers now have a party record, the Customer screen will show them as well.