grub: Block list syntax
13.3 How to specify block lists
===============================
A block list is used for specifying a file that doesn't appear in the
filesystem, like a chainloader. The syntax is
'[OFFSET]+LENGTH[,[OFFSET]+LENGTH]...'. Here is an example:
0+100,200+1,300+300
This represents that GRUB should read blocks 0 through 99, block 200,
and blocks 300 through 599. If you omit an offset, then GRUB assumes
the offset is zero.
Like the file name syntax (⇒File name syntax), if a blocklist
does not contain a device name, then GRUB uses GRUB's "root device". So
'(hd0,2)+1' is the same as '+1' when the root device is '(hd0,2)'.
Info Catalog
grub: File name syntax
grub: Filesystem