Rules Index
Player Core Chapter 8: Playing The Game Checks Step 3: Compare to DC

Getting a DC From a Modifier

Source Player Core pg. 393
When someone attempts a check against you, you might need to defend with one of your statistics that's normally a modifier, such as your “Reflex DC.” Rather than rolling a check of your own, you need to generate a fixed DC based on your modifier. Your DC for a given statistic is 10 + the total modifier for that statistic. So if you have a +4 Reflex save, your Reflex DC is 14.