The Rule action can be used for calculations that are nested IF / THEN / ELSE conditions. This action is similar to CASE statements in SQL. It can be used to create new columns as well as replace existing ones.
To calculate aggregates (grouped or not) such as sum or count, use the Aggregate action.
As you may have noticed by now, in EasyMorph the approach to calculations is different from Excel. The expression syntax is more restricting (which, by the way, is good for performance) and doesn't allow referencing values in other rows, tables or aggregations over ranges. Nevertheless, such calculations are still possible with the help of additional actions that pre-calculate/bring necessary components of a value calculation into the same row. For instance, the Shift columns action can be used to bring values from a previous or next row. Then they can be used used in an expression together with values from the current row which makes it equivalent to referencing a value in a previous/next row. Note that in EasyMorph it's generally not possible to arrange recursive calculations. Although in some cases workarounds are possible.
The Total/subtotals action creates a new column with an aggregated total, or group subtotal (if grouping was used). This action is different from "Aggregate" because it doesn't roll up rows, but instead just adds a new column with total/subtotals. It makes it possible to write expressions that use a row value and its column total (or group subtotal) in the same expression (e.g. calculation of the percentage from the max value).