/grub.d/README

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 2All executable files in this directory are processed in shell expansion order.
 3
 4  00_*: Reserved for 00_header.
 5  10_*: Native boot entries.
 6  20_*: Third party apps (e.g. memtest86+).
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 8The number namespace in-between is configurable by system installer and/or
 9administrator.  For example, you can add an entry to boot another OS as
1001_otheros, 11_otheros, etc, depending on the position you want it to occupy in
11the menu; and then adjust the default setting via /etc/default/grub.