List & Label distinguishes between two kinds of data fields: on the one hand there are data fields that are filled with content once per printed page (once per label or file card), these are called "variables" in the List & Label terminology. On the other hand, in a report, there are data fields that are filled repeatedly with different contents for a page, e.g. the data fields of an item list of an invoice. These data fields are called "fields" in the List & Label terminology.
For this reason, in file card or label projects only variables can be used, while in list projects both variables and fields can occur. For printing an invoice, an application would typically declare the invoice header data such as customer name and address as variables, while the item data such as article number, unit price, description etc. would be passed as fields.