Default Setting module is located at Setup > Company Details.
Each document type has a set of fields which are eligible for default setting, meaning that users may set those fields to automatically contain a specific number value, text value or tick; from the moment a new document entry begin. Note that the values in those fields may still be edited (or deleted) at document entry before creating the document.
The Default Setting Registration module is a consolidation of those eligible fields, separated into tabs according to the document type, and further according to activity.
Each document's tab also contains a Layout button (with a printer image icon), which contains the printing/preview properties of that document type, such as document layout selection, paper size & orientation, and other specifications.
Every document has the fields Reference 1 - 5, which are located at the "properties" tab at document entry page.
By default, these are just extra fields where users may store information relevant to the document; standard document layouts do not display values on these fields on the document printout/preview.
However, these Reference 1 - 5 fields can be utilised as extra display fields on customised document layouts.