Defaulting Heirarchy for Legal Entity

Hi, We are in the initial stages of a R12 implementation, determining our system organisation structure (Ledger, LE, OU's etc).

In the scenario where there is one Ledger, one OU and multiple Legal Entities, how does the system default the LE for any transaction in the subledgers (AP, PO and AR)? My understanding is that if the system can't work out a default, the user has to explicitly enter it. I would like to know the rules that are used to determine the default.

I know there is the default Legal Context on an OU but in the scenario of one OU and multiple LE, that will not be right in all cases. I assume the defaulting hierarchy could be different, depending on the transaction/module (e.g. AP invoice to AR invoice to PO Reqs?). Is the defaulting hierarchy documented anywhere? Can someone point me to it, please.