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

Can someone help explain the Slot A and Slot B stuff in TWRP? twrp slot a slot b 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 golden 777 slot real money Hello,I've successfully installed twrp and rooted via experimenting with TWRP, I tried booting from the 'B' slot Failure, stuck in fastboot

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