What is the 'the default user interface' referred to by the aShowDefaultUI parameter in element.execCommand()?

According to the API for the element.execCommand() function, it says that it has three parameters: aCommandName, aShowDefaultUI, aValueArgument.

The API's description of the first and third parameters is perfectly clear but I'm unsure about the meaning of the second parameter.

This is what the API says:

aShowDefaultUI: A Boolean indicating whether the default user interface should be shown. This is not implemented in Mozilla.

What is it referring to when it says 'the default user interface'?

For reference, I'm using element.execCommand() to create my own WYSIWYG web text editor that will only ever need to work in Google Chrome.