Suppliers are first created in header levels which are same for all the operating units. Next we create supplier sites specific to an operating unit. When we create a purchase order/invoice, we create it against a particular site. 

For customers we first create party, which is same across all the operating units. Next we create an account/customer . The customer number can be same/or differnt in different OUs. When we create a sales order/AR Invoice, we create it against a particular account/customer. Each customer/account can have more than one address (like supplier sites)  and each address(party site number) can function as a ship-to/bill-to/drawee.

In summary, Supplier headers are OU independent. Supplier sites are in operating unit level and represent different address. Party is OU independent. Customers numbers can be same/differnt  in operating unit level and customer sites represent different addresses.

Example
In the first picture the part name is 3M Health Care1(no ). This party has 3 different accounts – 2 in vision operation (4301, 4302) and 1 in vision Germany (4303). The account (customer) number 4301 has two different address (sites :  18038 – Bhubaneswar and 18040 Cuttack).

Below img shows the customer (account) 4301 with its two address(sites) and site number 18038 details