We have a custom field named "Region__c" in the account object. The same field is also present in the Contact object.
When we select an account form the contact edit view we want to populate the contact's Region__c field with the one present in the selected account record.
Simply we want to replicate what salesforce does with the address field: the contact MailingAddress field is automatically populated with the value of account's BillingAddress field when you select the account with the lookup search page.
Best Answer
It can be done easily using workflow rule on Contact Object.