Multi-Org or multiple organization access (MOAC) is basically an ability to access multiple operating units from a single application responsibility. Here below is the complete Logic of the HR security profile in oracle apps r12.
Prior to R12, end users use to toggle / switch / change responsibilities to do transactions (like invoice / payment processing in AP) in different operating units. This is a very time consuming and inefficient way of recording transactions when you have 100s of operating units specially Internet based organizations who have worldwide operations in almost all the countries.
To address this, a new feature in R12 has been introduced in which user can switch between operating units within a responsibility something like “Change Organization” feature in inventory. Prior to R12, user would have to switch responsibilities to enter transactions in respective operating units (tagged to the responsibility).