How is LE tied to OU in R12

In the Ledger setup the relationship between LE and OU is defined.

 LE is tied to Ledger in the following relationship   LE >- Ledger .  Multiple LE to one Ledger

 OU is tied to Ledger in the following relationship OU >- Ledger.  Multiple OU to one Leder

However the default LE Context in the Operating Unit Definition Screen allows you to choose what will be the default operating unit for all the transactions which you are going to make.

If you try to change the value of the LE context after setting up you get a message "APP-FND-02939 Changing the Primary Ledger after entering the subledger transactions for this operating unit may cause data corruption. Please create another operating unit if you want to use another primary ledger. " The metalink note corresponding to it says that as per development it is safe to ignore that message. But from user perspective I feel that it is not sufficiently descriptive.