This command toggles the dotted note duration for use during note entry. If it is selected when a note, chord member, or rest is added, the resulting item on the staff will have the dotted variation of its base duration.
The period on the keyboard can be used to toggle the dot attribute. If the double dotted note command tool appears next to the dot command on a tool bar, then the period will cycle through both dot attributes. You can use the Toolbar Customization feature to set this up.