In DotnetNuke Localization support, every resource key (in APP_LocalResources or APP_GlobalResources) should end with an extension.

resourceName.Text – Represents a string used for a common purpose
resourceName.Help – Used mostly in conjuncture with the DNN:Label control, which supplies a help icon () and tooltip.
resourceName.Tooltip – Used for a control’s Tooltip property.
resourceName.Action – Used to represent the Title property of a ModuleAction item shown in an actions menu.
resourceName.Confirm – Text that is displayed in a JavaScript’s confirmation dialog (such as a dialog to confirm deleting a module).
resourceName.Error – Text that is displayed as an error message. This might be for a label or, more commonly, for one of the ASP.NET validation controls.
resourceName.Detail – Text that gives more detail about an error message.

If we add a resource key in a resx file of the APP_LocalResources or APP_GlobalResources and didn’t give it an extension (like one of the above or any other one), we cannot get its value back by the DNN Localization logic of the:

DotNetNuke.Services.Localization.Localization.GetString(key, this.LocalResourceFile);

Or:

DotNetNuke.Services.Localization.Localization.GetString(key, <global resource file name>);

Further, if we call the Localization.GetString function with a key without an extension, it will assume that we are asking about the one with the extension ".Text".
So the call of Localization.GetString("SomeKey", LocalResourceFile) is equivalent to
Localization.GetString("SomeKey.Text", LocalResourceFile)!