Category: atom_site
Data type: char
Must appear in a looped list containing '_atom_site_label'
Permitted values:
. 'no constraints on atomic displacement parameters'
T 'special-position constraints on atomic displacement parameters'
U 'Uiso or Uij restraint (rigid bond)'
TU 'Both constraints applied'
A code which indicates the refinement restraints or constraints applied to the atomic displacement parameters of this site.