Thank you for your help, Chris! Figure 1 from Horn et al. (2017) is very informative as well.

Is there a way to transform SPM MNI coordinates (so smaller brain size; those are used to localise our Behnke-Fried Electrodes) to an arbitrary Atlas such as the subcortical system or the provided MNI brain? In a best case scenario I would also use that approach to verify that the subcortical system and the MNI brain have the same coordinate system. Or is my best bet to localize all electrodes again in the lead-dbs framework?