Clarification about fMRIprep coregistration procedure

I used fMRIprep 1.5.6 to preprocess the ADNI fMRI database; the fMRIprep report about coregistration is “Susceptibility distortion correction (SDC) was omitted. The BOLD reference was then coregistered to the T1w reference using bbregister (FreeSurfer) which implements boundary-based registration [Greve and Fischl, 2009]. Co-registration was configured with six degrees of freedom”.
I am a beginner in fMRI preprocessing. I need to know that coregistration is done using solid-rigid registration using a surface-based metric or an elastic method, like boundary-based registration methods.

6 degrees of freedom is a rigid registration (3 translation parameters, 3 rotation parameters). For details on boundary-based registration, please see the cited paper.