what does ${@:3} means in bash?

In the dist_train.sh from mmdetection3d, what does ${@:3} do at the last line ?

I can’t understand its bash grammar.

#!/usr/bin/env bash

CONFIG=$1
GPUS=$2
NNODES=${NNODES:-1}
NODE_RANK=${NODE_RANK:-0}
PORT=${PORT:-29500}
MASTER_ADDR=${MASTER_ADDR:-"127.0.0.1"}

PYTHONPATH="$(dirname $0)/..":$PYTHONPATH \
python -m torch.distributed.launch \
    --nnodes=$NNODES \
    --node_rank=$NODE_RANK \
    --master_addr=$MASTER_ADDR \
    --nproc_per_node=$GPUS \
    --master_port=$PORT \
    $(dirname "$0")/train.py \
    $CONFIG \
    --seed 0 \
    --launcher pytorch ${@:3}

>Solution :

It is standard parameter expansion:

${parameter:offset}
${parameter:offset:length}

This is referred to as Substring Expansion. It expands to up to
length characters of the value of parameter starting at the character
specified by offset. If parameter is @, an indexed array
subscripted by @ or *, or an associative array name, the
results differ as described below. If length is omitted, it expands
to the substring of the value of parameter starting at the character
specified by offset and extending to the end of the value. length
and offset are arithmetic expressions (see Shell Arithmetic).

[…]

If parameter is @, the result is length positional parameters
beginning at offset. A negative offset is taken relative to one
greater than the greatest positional parameter, so an offset of -1
evaluates to the last positional parameter. It is an expansion error
if length evaluates to a number less than zero.

The following examples illustrate substring expansion using positional parameters:

$ set -- 1 2 3 4 5 6 7 8 9 0 a b c d e f g h
$ echo ${@:7}
7 8 9 0 a b c d e f g h
$ echo ${@:7:0}

$ echo ${@:7:2}
7 8
$ echo ${@:7:-2}
bash: -2: substring expression < 0
$ echo ${@: -7:2}
b c
$ echo ${@:0}
./bash 1 2 3 4 5 6 7 8 9 0 a b c d e f g h
$ echo ${@:0:2}
./bash 1
$ echo ${@: -7:0}