This file is indexed.

/lib/partman/visual.d/20bootable is in ubiquity 2.18.7.

This file is owned by root:root, with mode 0o755.

The actual contents of the file can be viewed below.

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#!/bin/sh

cd $1

num=$2
id=$3
size=$4
type=$5
fs=$6
path=$7
name=$8

if [ -f $id/bootable ]; then
	printf "B"
fi