The red or green slider defines the 'design level of the nearest rail' (sometimes it is at the exact rail height, often it is a standard distance above, such as the 100mm shown on the picture I've linked to). The main 'offset' dimension shown is the horizontal distance between the plate and the running edge of the nearest rail. Track cant at the location is also recorded. So none of the dimensions actually show the platform height, but if the plate happens to be attached to a platform then the information will allow the track position with respect to the platform edge to be determined and maintained. The plates do not just appear on platforms though, but on any lineside structure where dimension to track is relevant to track maintenance.http://www.terram.com/products/signs-and-markers/railway-datum-gauge-sign.html
In that example, a horizontal line extended out from the red marker would be 100mm above the nearest rail - obviously the other numbers are just a 0-8 sequence so not typical.
The purpose of colour coded sliders is that a red slider can be re-adjusted by the maintainers to follow a gradually changing track position if movement occurs but it remains in tolerance with respect to lineside equipment - so a red slider might be found on an OHLE mast or signal post some distance from the track. However a green plate means that track position must be maintained in the absolute position shown. AIUI all modern installations alongside platforms should have green sliders.
A real life example and explanation of all the info is shown on railsigns.co.uk here: http://www.railsigns.co.uk/info/other1/other1.html