When you assign a reason code to a journal batch, the reason code is placed on all entries posted from that specific journal batch.
Go to the journal window (for example, the 
General Journal
window).
In the Batch Name field,
click the AssistButton 
to see the batch window.
On the line with the selected journal batch, click the
Reason Code field, and then
click the AssistButton to see the Reason Codes window.
Select the desired code, and then click OK.
Afterward, the new code will be copied to all journal lines in this journal batch.
Assigning Reason Codes to Journal Templates
Assigning Reason Codes to Journal Lines
Using Reason Codes when Invoicing