This information applies to versions 6 and newer
For RAW picture inputs, working color space is the native sensor color space for the first half of the image pipeline and then the DXO Wide Gamut colorspace, while older versions of DxO PhotoLab were using Adobe RGB. For non-RAW file inputs (TIF or JPG), the working color space is the one from the input file (unchanged).
To explain more about the DxO Wide Gamut color space, we have compiled some brief information in this link along with more details and background information in this other link.