Update define-calculated-fields.md

Author: neerajatmsft

powerapps-docs/maker/data-platform/define-calculated-fields.md

@@ -208,15 +208,21 @@ You should be aware of certain conditions and limitations when working with calc
 - Sorting is disabled on:  
   - A calculated column that contains a column of a parent row.  
   - A calculated column that contains a logical column (for example, address column)
-  - A calculated column that contains another calculated column.  
+  - A calculated column that contains another calculated column.
+  - A calculated column that contains `Now()` function.
 - Calculated columns can span two tables only.  
   - A calculated column can contain a column from another table (spanning two tables – current table and parent row).  
   - A calculated column can't contain a calculated column from another table that also contains another column from a different table (spanning three tables):   
     (Current table) Calculated Column ← (Parent Row) Calculated Column 1 ← (Parent Row) Calculated Column 2.  
 - You can't trigger workflows or plug-ins on calculated columns.  
 - You can't change an existing simple column to a calculated column. If your current application is using JavaScript or plug-ins to calculate a column, you wouldn't be able to use the calculated columns feature without creating a new column.  
 - Duplicate detection rules aren't triggered on calculated columns.  
-- A rollup can't reference a calculated column that uses another calculated column, even if all the columns of the other calculated column are on the current table.  
+- A rollup can't reference a calculated column that uses another calculated column, even if all the columns of the other calculated column are on the current table.
+- A rollup can't reference a calculated field that uses another calculated field, even if all the fields of the other calculated field are on the current entity.
+-  When a currency calculated field depends on a currency field from a related entity, we perform calculations using corresponding base currency field values. This is because exchange rate values and currency values can vary between different entity records. For example, consider a calculated field - `Account Revenue` on the `Opportunity` entity, if it has dependency on `Annual Revenue` currency field on the `Account` entity, the 'Account Revenue' is calculated as -
+
+    `'Account Revenue' = ['Annual Revenue (Base)' * 'Exchange Rate on Opportunity entity record']` or
+    `'Account Revenue' = [('Annual Revenue' / 'Exchange Rate on Account entity record') * 'Exchange Rate on Opportunity entity record']`
 
 ### See also