[Freesurfer] Unknown Label in following postprocessing with mne-python

Dear fellow colleagues,

in my study I use volumetric source spaces. In the process of creating them, I decided to identify all available volumelabels FreeSurfer "recognized" in the given mri image.

Every single subject inherits, despite all other volumelabels, a volumelabel called "Unknown". In an 3D representation of those "unknown" volumelabel coordinates I opine those points could also be part of other volumelabels like cerebral cortex or white matter.

Would you please give me a detailed explanation of the origin of this 'Unknown' label? Specifically in respect to the fsaverage "subject".

Best regards,

Daniel van de Velden