This checkbox can be found under Financial Relations > Transport > Transport Parameters.
If this option is checked, then during manual entry of loading/unloading addresses, only the addresses of the relevant client can be selected, and only those for which the "Delivery" checkbox is enabled.
For example, for De Efteling B.V., the option Limit Address Search is checked. If you then create a transport order with De Efteling B.V. as the client, you can only choose from the addresses associated with this client.
This also applies to emballage mutations, inbound/outbound orders, EDI transport orders, etc.