A little problem about partition coefficient

I have a little question and need help. In EE 7.1, toxics→toxics options→partition coefficients, what do “WC Partition Coeff” and “Bed Partition Coeff” mean?

I think the latter means the concentration ratio of toxics between in the sediment and in the water. But what does the former mean?

The WC partition and Bed partition co-efficients can be different due to the redox and organic carbon complexation differences between the WC and bed. However, if you do not have measured data or literature values for these then you can use the same values for WC and bed.