In K&D, if power-on scan is enabled 'Scan A' will only stop a scan for the timer B period. 'Scan B' lets you start and stop scan as desired.
Scan setting 'Power ON' tells the radio to start scanning when it comes on. In 'Scan B' the radio will not resume scanning unless 'Power ON' is on.
Moni selects Rx tone-off when the mic button is not grounded. I think OFF means tones are disabled off-ground but usually end up getting it wrong the first time. Ground the mic and watch for the moni icon to appear at the top of the display to verify.
F121's require that the mic button be grounded to scan. You can get around this by soldering a wire inside the mic from the hanger to ground to defeat the feature. You can also mod the front board, but the mic mod is easier. Newer radios (like the F5021) let you do the same in software.
When scanning, the radio will transmit on the 'Prio A' channel. Program a Px button for 'Prio A Rewrite' to change priority on the fly.
I avoid scan mode M2, and normally use M3 to check the Priority A channel during receive on other channels. If you use M2 you will transmit on the last received channel, not necessarily the one you intended to.
Stop the scan and press and hold the same Px 'Scan Start/Stop' button to select the scan list. Up/Down to the one you want, then press Px again. Changes made in one scan list don't change others. Blank out the rest of the scan lists so you don't have to scroll through them. 'Text ON' shows 'SCAN' on the display. Text OFF (blank) scrolls the display through the channels.
I use a 2~3 second 'Stop Timer' depending on the application. F121 scanning is a bit slow, and when priority sampling it will gap other channel reception. Nature of the beast, as Icom takes likes to make very sure the correct RX tone is being received before opening the tone gate. You can change the Scan timer settings in Common|Expert, but if you speed it up too much you will miss calls. Resume sets the amound of time the radio will wait before restarting a scan after reception.
Power-on beep is enabled by any entry in the 'Opening Text' field. No text, no beep.