[chimerax-users] Exact meaning of RMSF (RMSD in sequence alignment)

Dear All, I was not able to find out what is the formula for calculating RMSF in the commans Sequences-> Headers-> Calpha. Does it calculate the RMSD per residue of distance between C-alpha of each of the associated sequence and the displayed sequence, or RMSD per residue between all of the associated sequences? Thank you very much in advance for explanation. Best regards Sasha