The Bootorder parameter directly influences the BIOS setting for the bootorder.
Bootorder=N means that the first bootdrive is set to Nothing/disabled. Normally you have to enter three letters to reflect all three bootoptions.
Using only one will result in Bootorder=xNN. This means that the last one or two letters are always filled with “N”s.
If you do not want the unit to PXE at startup the best wouold be to use