So it looks like it's mentioned here:
Mellanox Breakout Cables 40G > 4x10G and 100... | Mellanox Interconnect Community
"
Overview
The Breakout cable is a unique Mellanox capability, where a single physical 40GbE port is divided into 4x10GbE (or 2x10GbE) NIC ports. It maximizes the flexibility of the end user to use the Mellanox switch with a combination of 10Gbps and 40Gbps interfaces according to the specific requirements of its network.
Important: The 40GbE split options is supported only on Mellanox switches and not supported on Mellanox adapters (e.g. ConnectX-3) when equipped with 40GbE ports. In case you wish to limit the 40GbE port on the adapter to 10GbE you can use QSA or similar 40GbE-10GbE cables (Refer to Mellanox.com -
here)
Split options:
Not all 40GbE switch ports can be split to 10GbE ports, certain ports cannot be split at all and there are 40GbE ports which can be split into 2x10GbE ports only. The ports that can be split into 2x10GbE ports, using lanes (connectors) 1 and 2 only. Connectors 3 and 4 are left unconnected. In addition, when performing the split, additional port may be unmapped and cannot be used. The map of ports that can be split per switch can be found in the Switch Hardware Manual located in Mellanox website.
Note: The maximum number of ports (mixture of 40GbE and 10GbE) is 64 ports."
Maybe it only applies to Mellanox adapters but I doubt it.
