Formula Linkbases and Link Roles
While standard XBRL linkbases (Definitions, Presentations, Calculations) define relationships between concepts via locators and standard arcs in standard extended links, a formula linkbase defines formula components (formulae, variables, filters, assertions, etc) and their relationships. These definitions are specified via resources and generic arcs in generic extended links.
Adding a formula linkbase
In the Overview entry helper (screenshot below), right-click the taxonomy file or an existing linkbase and select Add New Linkbase | Formula. The added linkbase will become the default formula linkbase file. The default formula linkbase file is the file into which new formula definitions will be saved when the taxonomy file is saved. If you wish to make another formula linkbase file the default formula linkbase, right-click it and select Set Default Linkbase | Formula (see screenshot below).
Note that default linkbases are displayed in bold and that linkbases that have been modified but not yet saved are marked with an asterisk.
The formula linkbase is displayed in the Formula tab.
| Note: | If a formula component is added to the taxonomy at a time when no formula linkbase exists, a formula linkbase is created automatically. |
Link Roles
As is the case with standard extended links (for Definitions, Presentations, Calculations), generic links must define an extended link role value, which partitions relationships of the same type into disjoint networks. All generic extended links with the same link role are combined under one link role node in the diagram in the Formula tab, even if they reside in different linkbase files.
Generic link roles can be created in the diagram via the context menu of the background area (screenshot below). Note, however, that this context menu will be displayed only if the View Option combo box of the Formula tab has been switched to Show All Extended Link Roles.
This menu is also available via the 
toolbar icon, Add Extended Link / Manage Linkroles. Since relationship networks are not that important for a formula linkbase, the default view of the Formula tab is Hide Extended Link Roles, which hides the link roles and, instead, shows the formula components without their link roles.
If there is no default formula linkbase file at the time the extended link role is created, a default formula linkbase file will be created automatically. And if there is no link role in the default linkbase file at the time a link role is created, then a link role will be created automatically in the default linkbase file.