“p.” or properties['<property-name>'] in alter, iterate, iterate_state, or alter_state commands can access or set user-defined properties that are atom-level (alter/iterate) or atom-state level (iterate_state/alter_state). atom-state means that these settings are set for a specific atom in a specific state.
# set a property for each atom: alter_state 1, all, p.xy = x + y # creates a selection foo that includes only atoms with x+y < 50 select foo, p.xy < 50
In the selection language, only the “p.” notation is supported.