Posting is the activity that updates the other record type.
Most tranactions are created through the Activities menu - although some are created internally by QuickBooks. All transactions are posted at the time they created - except purchase orders and estimates, which hold information used to create other transactions at a later date. Transaction documents contain header information that applies to the transaction as a whole - but no postable transaction data. Even though QuickBooks uses 24 different types of transactions, it stores them all in the same set of records. This is a flexible and powerful technique - but can difficult to understand.