IBM RS/6000 44P 270 Service Manual page 181

If more than one option is enabled, the system recognizes only the option
corresponding to the smallest menu number. For example, if options 4 and 2 were
enabled, the system recognizes only Option 2: Service Mode Boot from Saved List.
After a boot attempt, all enabled options are disabled. In effect, the system throws away
any menu options that are enabled after the option with the highest priority (the option
with the smallest menu number) is executed.
The user can also override the choices in the Boot Mode Menu while the system
initialization indicators display on the screen. For example, if the user had enabled the
system to enter the SMS menus (option 1) but pressed the 8 key while the system
initialization indicators displayed on the screen, the system would enter the open
firmware prompt and disregards the settings in the Boot Mode Menu.
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Chapter 7. Using the Service Processor