In the ribbon of the "Document Type" card is the button for Linked Tables. 

The "Linked Tables" page is used to link tables together. The purpose of this is to allow access to data that isn’t available from the Document Type’s primary table.

For example, a Document Type of “Sales Invoice” would use the Sales Invoice Header table to get invoice numbers; however, the Sell-to Customer’s email address needs to be retrieved from the Customer table as it is not stored on the primary table Sales Invoice Header.

To resolve this the tables are linked using the "Linked Tables" page. This means from the Sales Invoice Header table the Invoice number is related to the Sell-to Customer No. which in turn matches up to the Customer No. on the Customer table, which shows the email address.

From the Document Type page, select a document type and click Linked Tables. This will open a page like this:

The upper portion of the page is used to add tables that need to be linked to the "Document Type’s" primary table.

To add a new linked table simply click on the button labelled New. Fill in the field Link No., the link number must be a number from 1 to 10. Up to ten tables can be linked to the document type. Then populate the field Table No. that you want to link the document type with. This will automatically populate with the Table Name when a Table No. is chosen.

The lower portion of the page is used to configure the Links FastTab within each table that has been chosen. Select a link number in the upper portion and then change the settings in the lower portion. Effectively the lower portion is itself divided into two parts - the fields chosen from the linked table and the fields that are being linked to on the primary table.

The first field that shows is the Field No. These are the field numbers from the table that you are linking to the primary table. The Field Name will automatically populate with the Field Name when a Field No. is chosen.

The next field along is the Type field. This can be a Field or a Filter. Choose Field if linking to a field on the primary table. Choose Filter if using a filter field within your primary table or another linked table against the linked table above.

Note: if you are using type Filter then make sure to fill in the Filter field on the FastTab, which specifies a filtered value for the chosen field.

The Link -To field allows you to choose your Link-To type from Primary Table, Linked Table or Document Field. If you choose the option Linked table then you must populate the Link- To No. field. Dropping down on this field will give you a list of the linked tables you have selected on the upper portion of the page. Next fill in the Link-To Field No. use the assist (the three little dots on the field) to choose which field from your Link-To Type to use. Populating this field will automatically populate the Link-To Field Name based on what you have chosen.