Can someone help explain the Slot A and Slot B stuff in TWRP?

B partition ONLY)

I'm working with Android 9 Pie and I have one slot rooted and the other is not My reasoning to have my phone this way is

slot games with highest rtp Card slot microSDXC Internal 64GB 4GB RAM, 64GB 6GB RAM, 128GB 4GB RAM, 128GB 6GB RAM UFS MAIN CAMERA Camera 50 MP, f, 26mm (

slot pulsa gampang menang Boot TWRP in slot a; Install LineageOS , TWRP will install it to slot b! Flash to slot b