I created 2 fields on that is a summary field with a **total** ( FIELDNAME ) and another Calculated field and used the **SUM** aggregate function ..

Can someone explain which to use and when ... and in what ways are they different ?

I tried reading the FM help section but not sure when to use which technique ..

In my solution they are giving different values especially when working with a found set .... So I guess SUM and TOTAL are not the same !

the calculation sum() function allows you to summarize data from another table, and also add additional parameters to the calculation, EG:

If ( table::year = Year(Get(CurrentDate)) ; Sum(relatedTable::amount) ; "" ), say for instance you only wanted to show the sum amount for records of the current year.

The Summary Field Type will only return the sum from the found set, or sub-summarized set. Differences are (and should be) expected.

You haven't said what you're trying to do, just that they are different, so we can't recommend which to use.