Current keywords are cmenergies (in units of GeV), observables, phrases, and reactions. Individual keywords can be omitted if they are not relevant. Each keyword is associated with a list of possibly multiple values, e.g.

keywords: # used for searching, possibly multiple values for each keyword
- {name: reactions, values: [P P --> Z0 Z0 X]}
- {name: observables, values: [SIG]}
- {name: cmenergies, values: [7000.0]}
- {name: phrases, values: [Inclusive, Integrated Cross Section, Cross Section, Proton-Proton Scattering, Z Production, Z pair Production]}

Each reaction should consist of initial- and final-state particles separated by -->, with particles on each side of the arrow separated by spaces. The right-hand side should usually have an X to indicate an inclusive reaction (as opposed to exclusive where all final-state particles are known, e.g. elastic proton scattering). A standard notation should preferably be used for each particle. Omit the decay products or give them as a separate reaction.

Each observable should consist of only a single word, for example, use SIG for a cross section, DSIG/DPT for a differential cross section, N for number of events, etc., again preferably using a standard notation.

Each phrase should be preferably be selected from a list.

Note that the linked lists of particles, observables and phrases were created several years ago from the old HepData site and may not be relevant. Assigning keywords unique to a particular HEPData record is of little value, therefore keywords should be omitted if in doubt. It is an open issue (HEPData/hepdata#60) to make it easier to select keywords by developing appropriate software.