Packet Size - Variable
| IDN | Description |
| 1-3 | x, y, z parameters in fractions of unit cell |
| 4 | individual isotropic thermal parameter U |
| 5-10 | anisotropic thermal parameters U11, U22, U33, U12, U13, U23 |
| 11 | population parameter |
| 12 | anomalous population parameter |
| 13 | neutron scattering factor |
| 14 | atom label (24 characters) |
| 15 | residue code (4 characters) |
| char 1: remoteness indicator | |
| char 2: branch designator | |
| char 3: sub-branch designator | |
| char 4: alternate location indicator | |
| 16 | sequence code (4 characters) |
| chars 1-3: least significant digits of residue sequence number | |
| char 4: residue code | |
| 17 | data set number |
| 18 | chain code (4 characters) |
| chars 1-3: chain identifier | |
| char 4: most significant digit of residue sequence number | |
| 19 | residue name (4 characters) |
| 20 | atom serial number for Protein Data Bank |
| 21 | atom multiplicity for atoms in special positions |
| 22 | scattering factor pointer as a packet sequence number ofscat |
| 23 | thermal displacement parameter type (0=overall;1=iso; 2=aniso) |
| 24 | atom-group key for group refinements |
| 25 | model-refinement key for refining different model |
| 26 | number of valence electrons - added in MAPXCH |
| 50 | specified coordinates flag (0=no; 1= geometry fixed; 2=dummy) |
| 51 | atom label of attached atom (for geometry fixed hydrogen sites) |
| 52 | number of attached hydrogen atoms not present in atom record |
| 55 | refinement restrictions associated with this site (8 characters). |
| String may be made up of the following flags: | |
| S special position constraint | |
| G rigid group refinement | |
| R riding atom site attached to non-riding atom site | |
| D distance or angle restraint | |
| T thermal displacement constraint | |
| U Uiso or Uij restraint due to rigid body | |
| P partial occupancy restraint | |
| 56 | special information about this site (40 characters) |
| 57 | description of constraints to site parameters (40 characters) |
| 58 | description of restraints to site parameters (40 characters) |
| 101-103 | sigma x, sigma y, sigma z |
| 104 | sigma Uiso |
| 105-110 | sigma of U11, U22, U33, U12, U13, U23 |
| 111 | sigma of population parameter |
| 112 | sigma of anomolous population parameter |
| 113 | sigma of neutron scattering |
| 210 | atom number in dictionary dict |
| 212 | pointer to atom number in atom of the back chain |
| 213 | pointer to atom number in atom of the furthest forward chain |
| 214 | mobility indicator |
| 215 | KEY for group/non-group/slave |
| 300 | 6-char code for geometric constraint (same as CALCAT) |
| 301 | Bond distance to parent atom site |
| 302 | Alpha 1 |
| 303 | Alpha 2 |
| 310 | 24-char label of sites used in the geometric constraint |
| 311 | Encoded symop/translation (if not 1 555) |
| 312-317 | as for 310 and 311 for three additional site specifiers |
| 320-323 | 24-char label of sites in the same rigid group (eg. methyl) |