In the design page, you can drag the "Form Template" item in the left panel and drop it into the center panel. It will allow you to link form field with another form in the grid format. A grid or inner table will be popuped when the link field is clicked.
Click "Use data types from the existing form" button, a popup window appear which let you select the linked form. Once a linked form is selected, you will be able to open linked form which you can set the configure for the linked form.
In the page, you can choose Select only or input only. Select only means you will be able to select the existing data and link them to the field. Input only means you can fill the linked form. You can also set the required field, print (means appear during print), or show/hide field.
You can set the display field which will be appeared in the main form. If it is number, you can also choose format option such as: sum, count, average, summary etc.
See the following image for the listed grid data when user clicks the linked field.