When working with balances tables such as the Account Balances (F0902) or Asset Account Balances (F1202), it can be a challenge to work with the amount fields (AN01 through AN14). Rather than deal with 14 amount columns, it would be much simpler if you could combine all the columns into one column. This is what the Period Amounts calculations does.
So what are the advantages of using Period Amounts over say summing the amount columns manually? Plenty:
Easier to change the period value: Any time you want to change which period or which through period, just change the value of the Period Amount calculation. If you manually summed the columns, then every time you needed a different period, you would have to delete the old sum, re-select the columns you wanted to add, and add the new sum calculation
Easier to build a report foundation: With Period Amounts you only have to create one calculation. This means that any calculation you need to do off the period value can reference the one calculation. This makes maintaining and building your amount-based report much more simple and maintainable.
Hook up to Report Parameters: You can hook up the Period Amount parameters to your Report Parameters. This lets you prompt the user for the period number when they run the report.
To add a Period Amounts column:
Open a table or view with AN01-AN14 (such as the Account Balances or Asset Account Balances tables)
Click the 
Calculation Editor
button on the main toolbar
Type in Period Amounts under the Type of Calculation
On the Parameters, enter 1 for Begin Period. Enter any number from 1 through 14 for End Period.
Note that if Begin Period and End Period are the same number, then the calculation represents the amount for that period. If Begin Period is 1 and End Period is another number, then the Period Amount is the "through" period amount (e.g. such as Year-To-Date depending on the year start period).