-i <vendor id> is used to specify a custom USB vendor id
according to the fastboot --help page
usage: fastboot [ <option> ] <command>
commands:
update <filename>
reflash device from update.zip
flashall
flash boot, system, vendor and if found,
recovery
flash <partition> [ <filename> ]
write a file to a flash partition
erase <partition>
erase a flash partition
format[:[<fs type>][:[<size>]] <partition> format a flash partition.
Can override the fs type and/or
size the bootloader reports.
getvar <variable>
display a bootloader variable
boot <kernel> [ <ramdisk> [ <second> ] ] download and boot kernel
flash:raw boot <kernel> [ <ramdisk> [ <second> ] ] create bootimage and
flash it
devices
list all connected devices
continue
continue with autoboot
reboot
reboot device normally
reboot-bootloader
reboot device into bootloader
help
show this help message
options:
-w
erase userdata and cache (and format
if supported by partition type)
-u
do not first erase partition before
formatting
-s <specific device>
specify device serial number
or path to device port
-l
with "devices", lists device paths
-p <product>
specify product name
-c <cmdline>
override kernel commandline
-i <vendor id>
specify a custom USB vendor id
-b <base_addr>
specify a custom kernel base address.
default: 0x10000000
-n <page size>
specify the nand page size.
default: 2048
-S <size>[K|M|G]
automatically sparse files greater
than size. 0 to disable
As for usage, I've never had to use it, but this suggests that it's for getting fastboot to work with unrecognized devices:
As you've already know, you can force fastboot to work with a device, even if the vendor ID is unknown by fastboot binary, by -i parameter: -i <vendor id> specify a custom USB vendor id