Thank you @Francois. Indeed, I wanted to compute the volume head model so I guess I'll stay with the Unconstrained option.
I have one more question if you don't mind 
I am working at the moment on estimating the epileptic brain activity at the source level. I have used the volume head model to estimate the activity sources using MNI, dSPM. Afterwards, I want to project my sources onto the template grid and compute the connectivity.
However, the source estimation using unscaled dSPM shows very high levels of synchronisation when visualised. I see that this is probably simply because the activation scale is computed for each window. Therefore, I decided to normalise the results and I computed the scale-average dSPM. The source estimation looks great now as the activation scale is normalised.
I am confused however by the message I got when scaling the dSPM source estimation:
Do I understand correctly that the resulting scaled dSPM cannot be projected onto the template grid then used for connectivity computation (since it can be used for visualisation and interpretation only)?
Apologies if this question is trivial, I googled it before deciding to ask. Thank you so much again for your invaluable help.