block list of grub (e.g. chainloader +1)

http://www.gnu.org/software/grub/manual/html_node/Block-list-syntax.html

11.3 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 (see 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)’.