These are the "Roll-Off" points for the Mid Range Driver's Crossover settings.
In a Nutshell, the "1.5 kHz" is the Low-End Rolloff Frequency for the Mid Range Driver, as set by the Crossover design; and the "6 kHz" is the High-End Rolloff Frequency.
The Crossover will "Bandpass" the range between 1.5 kHz and 6.0 kHz through the Mid Range Driver (the "middle" Speaker in each Enclosure).
Frequencies < 1.5 kHz become attenuated at the Crossover, and will be Low-Passed through the Woofer Driver - via the Low Hz Crossover section (if included...).
Frequencies > 6.0 kHz become attenuated at the Crossover, and will be High-Passed through the Tweeter Driver - via the High Hz Crossover section.