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src-dst-mac uses both the frame's source and destination MACs for selecting a link. src-dst-ip combine uses both the packet's source and destination IP for selecting a link.
As often one end is the same device, e.g. a gateway MAC or a local LAN host, using the combination of src-dst, often offers some variability to the hash.
Both generally "fail", when there are multiple flows between a pair of hosts. For that situation, some devices can include L4 port numbering as part of the hash.