The basic filtering method filters companies by the exact value of the Relationship Type field.
"adresar/(typVztahuK='typVztahu.dodavatel').json" -> Supplier
"adresar/(typVztahuK='typVztahu.odberatel').json" -> Customer
"adresar/(typVztahuK='typVztahu.odberDodav').json" -> Customer/Supplier
The second filtering method filters companies if they can be a customer or a supplier.
"adresar.json?filter[typVztahuK]=typVztahu.dodavatel" -> Supplier + Customer/Supplier
"adresar.json?filter[typVztahuK]=typVztahu.odberatel" -> Customer + Customer/Supplier
"adresar.json?filter[typVztahuK]=typVztahu.odberDodav" -> Supplier + Customer + Customer/Supplier