fir compiler: "actual coefficients" or "bit width only"? scaling and SNR relationship

Hi,

I'm just beginning a filter design and have some questions regarding the "fir compiler" core. In the parameterization GUI what is the difference between "actual coefficients" and "bit width only" ? Why is the resolution of the output lower when I choose "actual coefficients" ?

Can anyone also explain me this snippet from datasheet ?

Auto scaling (without the power of two option) provides the maximum signal-to noise ratio. All other scaling factors such as Signed Binary Fractional can result in a loss of effective bits (that is, where each effective bit provides 6dB of SNR).

thanks a lot.