Synchronous v/s Asynchronous mode

Now that I am undergoin trainin on BDC-Batch Data Communication...I came across the three update modes that are available in Call Transaction Method- Synchronous processing,
Asynchronous update and Local update.The Difference btw Synchronous and Asynchronous is often misinterpreted. So I thought I ll give a clearer picture by explainin with examples.

If the update mode is Synchronous: Till the previous record is not updated in the database, next record is not implemented.
Scenario:Lets say we do a bdc recording to create a customer and a sale order.First, a customer is created and then a Sales Order is created for him.
#1 If we give the update mode as Asynchronous update, the process of creating a customer starts and irrespective of whether the customer is created, the Sales Order creation for the particular customer starts. Therefore, there may arise a situation where a customer is not yet created, but a sales order is bein created for him.
#2 To avoid the situation cited above, we have to use Synchronous processing mode. Here, what happens is that a sales order is not created for a customer 'A' unless the customer 'A' is created. i.e., transaction creating the sales order waits for the transaction creating the particular customer to be completed first.

So when do we have to use Asynchronous mode?
Use this mode when there is no dependency between two transactions.For example, you are creatin a customer. Here, there is no harm even if the customers are not created in the
order that is given in the flat file, since each customer is a different entity.
As far as possible, try to use Asynchronous mode because it is faster.If you dont mention any mode, Asynchronous is the default mode.
The third mode is the Local update. Since I did not understand this mode, I request the visitors to throw some light on the working of this mode...through their comments. Thanks in advance..