Document type reminder in Gen Journal and Currency form

bangswit

There is several Document type in general journal, such as : Payment,Invoice,Credit Memo,Finance Charge Memo,Reminder,Refund
my question is.... what is the function for document type Reminder ??

and in form currency exchange rate (Form 511) there
Currency code
Exchange Rate amount
Relational Exchange Rate
Relational Currency Code
my question is.... what is the difference between the left one and the right one?

Thanks