I think PD_CALIBRATION should stay, but be updated a bit.

The pupose of PD_CALIBRATION is to provide a free-text method of stating the equation used to convert between channel or position and 2\q/E/d/TOF/whatever. Given free-reign to rename it, it would probably be PD_CALIB_XCOORD_EQUATION, but that's (I think) an unnecessary complication.

It isn't necessary to be machine-readable, as you can just give the conversion directly in the data listing, as

loop_
_pd_data.id
_pd_meas.detector_id  # could also be _pd_meas.position
_pd_proc.energy_detection
_pd_meas.counts_total
a   1    50000    1234
b   2    50100    1434
c   3    50200    1457
#...

From the given data, the conversion is given on a on-to-one basis.

.

The category currently consists of

• _pd_calibration.conversion_eqn, and
  • a text description of the equation used to convert from channel/detector_id/position to 2\q/E/d/TOF/...
• _pd_calibration.special_details
  • a description of special stuff about the calibration equation

The category delegates the task of recording the source of the calibration information to _pd_calib_std.external_block_id.

I would expand the category by adding

• _pd_calibration.diffractogram_id
  • Category key, and the diffractogram ID of the diffractogram to which the conversion applied. If there are multiple detectors or some such jiggery pokery that adds complexity, well, the equation is a free-text field, so you can write what you want.
• _pd_calibration.ref_diffractogram_id
  • The diffractogram ID of the reference diffractogram from where the calibration information was determined.