If your RAM is single channel, all data is flowing along a single memory bus that can handle 64 bits of data at once. If it’s dual channel RAM, the data is flowing along two 64-bit routes at the same time. Most modern gaming motherboards come with dual-channel mode support. Similarly, most modern CPUs support dual-channel memory.
Joking aside, yes, you don't need identical sticks of RAM in all four slots. I am not 100% certain, but I think as long as you use the same amounts in each channel you can even still run them in Dual Channel.(e.g. A 8GB and 16GB stick in Channel 1, and then the same in Channel 2, rather than both 8GB in Channel 1 and both 16Gb in Channel 2).
In usual case (when manual doesn’t have other explicit instructions) slots must be filled in following order. First slot of first channel (A1 in example) First slot of second channel (B1) How to correctly install computer memory. The memory modules are installed into matching banks, which are usually color-coded on the motherboard.
8tlH.