In the Summit Hardware Installation Guide
for EXOS 21, the "Stacking Cables" table is incorrect with respect to stacking-cable support on the X440-G2 switch.
At the bottom of the table, the stated limitation -- 10G stacking ports on ExtremeSwitching X440-G2 (48-port switches only), Summit X450-G2 10G, and ExtremeSwitching X620 switches -- should apply only to the 10 meter cable (model no. 10307). The shorter cable lengths (model nos. 10304, 10305, and 10306) are supported for all
models of the X440-G2 as well as for the X450-G2 (10G models) and the X620.
I'll be sure to correct the table in the next update to the document.