How does it work? Is it based on your contributions as at the date of the request or is it presumed you will continue to contribute until your retirement date??
It's both. It gives both figures. The forecast at retirement date makes it clear it you need to continue contributing to get that figure, and it also says what youve accrued so far
See the example on buenchico's link
The forecast is what you will get with your contributions to date. The optimum figure is what can get and how many years of contributions it will take to get to that figure.