flex memory

Can someone who knows stuff about dual channel and flex memory mode explain to me , what's the difference between them ? I've read a comment somewhere, where someone said "single channel < flex memory mode < dual channel". I am going for a laptop which will have 8GB + 4GB RAM and want to know...

I bought a HP Probook 450 G6 recently and it comes with 4GB DDR4 RAM. I plan to add a 16GB stick in the other channel and I am aware the board supports a maximum memory of 32GB RAM. I did some research on having different RAM sizes and I found out that Intel's FLEX MEMORY feature allows...