A Data Column is used in a Visual Invoice or Report template to show one specific variable for all associated items on the invoice or report. For example, you can have an "ItemDate" data column, which will look like this when editing your template:
When you create an actual Invoice or Report using the template, the Data Column will fill in information related to the items on the Invoice or Report based on the selected variable. This specific data column, for example, will show the dates for all items in the invoice or report:
The idea is that you should line up multiple data columns next to each other on your template (such as ItemDate, ItemName, ItemAmount), and it will then appear as a table of information. You can alternate text color and/or background color to make the invoice template more visually distinguishable. You may also setup the row height and row spacing to change the size and space between items. Keep in mind, you will need to change these settings for each data column if you want all data columns to look similar.
Another method for showing variable information in visual templates is by using Data Rows.