grub/util/grub.d

All executable files in this directory are processed in shell expansion order.

  00_*: Reserved for 00_header.
  10_*: Native boot entries.
  20_*: Third party apps (e.g. memtest86+).

The number namespace in-between is configurable by system installer and/or
administrator.  For example, you can add an entry to boot another OS as
01_otheros, 11_otheros, etc, depending on the position you want it to occupy in
the menu; and then adjust the default setting via /etc/default/grub.