To help with negotiations and get you close to previous salary, the government may require a pay stub or W-2. Only base salary will generally be considered; not bonuses, commissions, stock compensation, or hourly consulting.
What if I want more than my previous salary?  Or is salary always locked to your last paystub?  I'm not super familiar with the public sector but I've never heard of such a thing as requiring paystubs from your previous employer as a part of an application process.  That wouldn't even be legal in California.
Oh government....you're so confused...