L600 “height input only”

I just purchased a pair of L600. On the back there are 6 total binding posts. I understand that the 4 grouped together are for bi-wiring or bi-amping. But the top 2 have a sign that says: “height input only.” What does that mean? There is nothing in the owner’s guide to explain it.
Comments
Panamax 5400-EX
Sunfire TGP 5
Micro Seiki DD-40 - Lyra-Dorian and Denon DL-160
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Ft - SDA 1C
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https://www.crutchfield.com/S-kmtPZUMKXMJ/p_107L900B/Polk-Audio-Legend-L900.html
2. JM Labs Electra 920.1; Parasound JC2 and JC5; Sony HAP-Z1ES; Marantz SA8004 SACD; Music Hall MMF7 and Acoustech phono pre; PS Audio P1000; MIT S1 Cables
3. Polk LSiM703; Anthem Pre-2L SE; Parasound A21; Sony 48" 4K TV; Wyred4Sound DAC 2; Oppo 203; Squeezebox Touch; PS Audio Power Director 4.7; MIT S3 cables
2. JM Labs Electra 920.1; Parasound JC2 and JC5; Sony HAP-Z1ES; Marantz SA8004 SACD; Music Hall MMF7 and Acoustech phono pre; PS Audio P1000; MIT S1 Cables
3. Polk LSiM703; Anthem Pre-2L SE; Parasound A21; Sony 48" 4K TV; Wyred4Sound DAC 2; Oppo 203; Squeezebox Touch; PS Audio Power Director 4.7; MIT S3 cables