Each Document type is given a unique Code, this code can be found in multiple locations within Microsoft Dynamics 365 Business Central. The code is also given a Description, so you know what type of document you are going to be running. The Primary Table No. Filter field tells the system which record(s) this document type should be used with. For example, a Sales Order Confirmation document type should use table 36 (Sales Order Header), while a remittance advice document type to be printed from the Payment Journal page should use table 81 (Gen. Journal Line).
In addition, the Primary Table Document Type should be set when using table 36 (Sales Header) or 38 (Purchase Header). For example, sales quotes and sales order confirmations (both of which use table 36) it will allow you to choose a separate primary table document type.
NB: If you try to set the Primary Table Document Type when you are not using table 36 or 38 then it will error.
The Group by Field No. allows you to set a grouping at document type level, allowing you to highlight multiple records and for the app to determine based on your setup how the records should be sent out. For example, if you group by Sell-to Customer No. then all records highlighted to be sent will be grouped by this field. This means that your customers will not receive multiple emails for individual records, instead they will receive potentially one email with multiple records attached.
When you click on the assist next to the field you are given all the fields that you can choose from to Group by, these are fields based on your Primary Table No. Filter
The Group Field Name simply shows the name of the field you have chosen to group by.
Choosing Preview Before Sending tells the app that for this document type when you choose the option to Send Email (detailed further on in the user guide), the preview screen will pop up detailing the records you are sending, who they are being sent to and how they are being grouped.