Hi Jian,
CTA is not only related to adjustment of currency difference but It refers to the full process of currency translation i.e.- Required dimension properties , Business Rules, Script Logic , Loading exchange rate , Loading financial data , Executing currency translation task.
The document i.e. CTA you are searching in the web is not going to be find by you because it totally depends on business requirement. It contains logic regarding how you want to adjust the currency difference which is generated after currency translation, the difference which you want to adjust by manual input , journal posting or you post the difference value to some other GL.
Khokan