What is the difference between DRAM channel and DRAM Rank?
A DRAM Rank is a set of memories associated with a Channel (controller) that share a common address and data connection. Multiple Ranks can be connected to a Channel, but only one Rank can be activated at a time.

For example, if you have a 64-bit controller with two 64-bit DIMM slots, each slot is a separate rank. Only one DIMM is active at a time.

It is also possible to have multiple Ranks on a single DIMM. Nevertheless the principle is the same: one or more groups of physical chips, activated one group at a time.

A DRAM Channel is a controller interface that can talk to one or more Ranks. It is a common group of address / data lines that function together.

On devices have more than one DRAM  Channel, the Channels can be treated either as separate address spaces, or aggregated together to create a wider interface.

When the Channels are aggregated together as a wider interface, the memories on each Channel must be the same size and type. If they are treated as separate spaces, there is more flexibility in how the memories are configured.