I think what you are asking is the difference between ferrite number and ferrite content. Ferrite content is measured during welding procedure qualification testing. Basically a macrosection is prepared and  the weld metal and heat affected zone have their phase balance measured by a point counting method  (ASTM E562). The other test uses something like a magnetic instument (ferritscope) which can be used to assess the amount of ferrite present in production welds. Welding variables that affect their values are mainly welding consumable/process selection, welding heat inputs and control of interpass temperatures. The last two being a major influence on thinner sections especially on the higher alloyed grades of duplex.