The Set Variable action is used to set the values of the selected workflow variables. To configure the Set Variable properties, select the action and click the "Settings" button under the set variable action, the properties windows will be displayed on the right of the workflow designer.
Chose from "Current approval form" or " Another approval form".
If chose " Another approval form"
Application: From the dropdown list, select the application which the data source come from. By default, the current application is selected.
Data source: Select the data source from the dropdown list. The data source can be a data list, a document library, a form report, or a data report.
Click the "Settings" button to open the pop-up window.
Click the "Add" button to add a new row. Select the variable from the left dropdown list, click the icon from the right input control to open the Expression Editor. Set the value as with static or dynamic value.
Click "OK" to save your settings.