Both formuale, ExtendLinear and LinearExtend, are used to extend the series. With similar names and purpose, they can be easily confused but they extend the series in different way.
ExtendLinear uses linear interpolation to extend the series between the latest value and a manually specified value in the future, thus extending the series with a trend.
LinearExtend is using the Least-Square method to create a fitted line, which in turn is used to extend the series to a specified point in the future.
In below chart, you can see the difference between two methods of extending the series: