Whenever we set up an Account in Financial => Cards => Account, We’d go to Account Currencies window and assign the Multi currencies in case the account will have transactions originating in currencies other than Functional Currency. Besides assigning the Multi currencies here, it also involves to check whether we need those currencies to be revalued or not. If Yes, we need to mark the Revalue Account option and then choose to Post Results to Account or Financial Offset Account. Here, Results mean the the Realized gain / Loss or the Unrealized Gain / loss as the case may be, resulting out of Revaluation of Multi currencies in the Financial Routines ( Financial => Routines => Revaluation)

The basic difference between choosing Post results to an offset A/c and post results to Account is If you choose the Offset a/c, the revaluation results would be posted to a separate account that is defined as Financial Offset A/c in Posting Accounts setup (Administration => Setup => Posting =>Posting Accounts). On the contrary, If you opt to post the revaluation results to Account instead, the revaluation results would post directly to the same Account. For example, You have an account "000-1100-00" assigned a MC US$ and you marked Revalue, Post Results to Account. So, When a Revaluation occurs, the gain/loss would post to 000-1100-00 itself. 
Let us consider three accounts for our example.
1. Posting A/c - Cash (CAD) A/c - 000-1100-00
2. Posting A/c - Financial Offset A/c - 000-1199-00
3. Posting A/c Realized Gain/Loss A/c - 000-9999-00(let us use the same as Unrealized Gain/Loss as well). 
4.Consider USD as your functional Currency. 
5.Consider we posted a GL Entry on 01/01/2009 for CAD1000(Exchange Rate - 0.50) into Cash(CAD) A/c. Now, we want to revalue  Cash(CAD) A/c as of Today 03/07/2009 (Exchange Rate - 1.00). So, Realized Gain in this case would be (CAD1000*1.00 - CAD1000*0.50) = USD500 Gain.

Option1: Assume we set up the Cash(CAD) A/c to Revalue A/c to Account - USD500 gain would be posted as follows

000-1100-00 A/c     Dr   US$500 (Cash (CAD) A/c)
       To
000-9999-00           Cr   US$500 (Realized Gain/Loss A/c)

Option 2: Assume we set up the Cash(CAD) A/c to Revalue to Financial Offset A/c - USD500 gain would be posted as follows

000-1199-00          Dr   US$500 (Financial Offset A/c )
       To
000-9999-00         Cr    US$500 (Realized Gain/Loss A/c)