What makes this particle unique?

The new particle has a pair of charm quarks and an up and down antiquark. Previously, several tetraquarks have been discovered, but this is the first with two charm quarks without charm antiquarks to balance them. Physicists consider the particle an “open charm,” noting particles with the charm quark and anticharm antiquark have “hidden charm” because the charm quantum number for the whole particle adds up to zero.