Yes, you need to enter Cont/Exp coefficients for all flow regimes, but they are filled in as 0.1/0.3 by default in the cross-section data. I suggest that you review Example 1, Critical Creek, in the HEC-RAS Applications Guide (find it under Help on the RAS menu bar). It shows you how to create a basic RAS model.
In steady flow, yes, you have to have contraction and expansion coefficients. If you have supercritical flow, you'll want to reduce those coefficients by an order of magnitude (example, if you are uinsg 0.1 and 0.3 for subcritical flow, then you should use 0.01 and 0.03 for supercritical flow.
When using unsteady flow, momentum changes are captured in the St. Venant equation of conservation of momentum, so you do not have to explicitly add in contraction and expansion coefficients. There is, however, a separate table for unsteady contraction and expansion coefficients, that you can use (but it is optional) if you feel you need more energy loss from contraction and/or expansion.
i checked example applications and all of them,the unsteady ones, had used 0.1 and 0.3 for the coefficients.
where is that separate table for unsteady contraction and expansion coefficients, thad you mentioned?
Even though you see 0.1 and 0.3 in the unsteady examples, those values are only there to be used if that geometry file is used in steady flow. Those values are ignored in unsteady flow. If you want to include contraction and expansion coefficients in unsteady flow, you must enter them separately. You can find that table in the geometry schematic, under the "Tables" menu item, "Contraction/Expansion Coefficients (Unsteady Flow)".