Fati,
I may be misunderstanding your question, but the scale in the print widget uses the scale of the current map extent. That is what is displayed in the text box. Checking the box will force the print widget to output the map at the scale listed. If the box is unchecked, the map will generally be whatever you see in the map viewer window. In other words, if the box is checked and the scale listed is a larger scale than what you are currently seeing in your viewer window, the print result will be whatever the layout can show at that scale, which may or may not be what you expect.
If you are using cached map services, the scales available will be those that you have built into your cached map. If you want to control the scales that users have available to print, you likely will need to create those scale levels as part of the cache. As an example, I have scales in my cache that will output common scales that our staff uses in their work (e.g. 1" = 40'). As long as they are at that scale on the zoom ladder (or enter that specific scale manually in the text box in the widget) PLUS choose a layout size that can accommodate the extent of everything they want on the map (i.e. letter sized layouts do not show much if you are using scales such as the 1" = 40' example above).
If I misunderstood what you are asking, please give us more details regarding what you are wanting to do.
