When customizing a sales order form in NetSuite under the Linked Forms tab, the transaction types that may be assigned are those that logically follow a sales order in the transaction lifecycle. These include "Invoice" and "Cash Sale". An invoice is generated as a billing document issued to customers requesting payment for products or services delivered. A cash sale, on the other hand, is used for immediate payment transactions, where payment is received at the same time as the delivery of the product or service. Both transaction types are directly linked to sales orders as they represent different paths a sales order can take towards completion of the sales process. "Journal Entry" and "Credit Memo" do not directly follow from a sales order in the same way and are thus not typically assignable in this context.References: NetSuite Help Center documentation on customizing transaction forms and the sales process workflow.