What is the difference between iso and img?

I haven't been able to check if there are any specifications, but generally

• .iso files with extensions are image files for optical media such as CDROM, DVDROM, etc.
• .img files with extensions are SD cards, USB disks, and other disk image files

I think it's used differently.

For .iso, see
for ISO image for English Wikipedia. There is a description on the page.

Quoted from Wikipedia:

An ISO image is a disk image of an optical disk.In other words, it is an archive file that contains everything that would have been written to an optical disk, sector by sector, including the optical disk file system.
ISO image files bear the .iso filename extension.

# Based on historical background, the output file was named cd.iso in the mkisofs documentation, so it has become popular.

Quoted from documentation:

To create a vanilla ISO-9660 filesystem image in the file cd.iso, where the directory cd_dir will become the root directory if the CD, call:

%mkisofs-ocd.iso cd_dir

The .img file, on the other hand, has English Wikipedia's IMG (file format) stating "originally floppy disk image file extensions."

Quoted from Wikipedia:

The .img file extension was originally used for floppy disk raw disk images only.

However, since floppy disks are no longer used, the meaning of raw disk images is now misused to refer to all bootable disk image files.

I think .iso means ISO9660.
I think .img is an image file in which the byte column of the device is written directly to the file.

I think it refers to the same file format.

In UNIX-based systems, extensions have little meaning, so think of them as just part of a file name.Therefore, it is not possible to judge by the extension.

However, .img is an image file of some kind of device, and it can be interpreted in a broader way, so I think that kind of interpretation is possible.

• ISO file is the archive format of optical media defined by the International Organization for Standardization (ISO).

• The IMG file is a format in which backups that begin with floppy disks are organized into a single file.It is now also used to create image files such as hard disks.

The ISO file is an archive format defined by the International Organization for Standardization (ISO) for optical media.

An IMG file is a format for backing up a floppy disk into a single file.It is now also used to create image files such as hard disks.

I have the impression that ISO is a collection of optical disks such as CDs, and IMG is a wider backup image (including CDs).

Distributions such as Ubuntu in the question basically distribute ISO images on CD/DVDs, but distributions for Raspbian and Raspbian for Raspi use the SD card, so they are distributed as IMG files.

Recently Ubuntu has adopted a format called Hybrid ISO that can be written directly to USB.

Also, the IMG file was mentioned below on the reference site.

There is only one version of the ISO format, but there are two versions of the IMG.
Compressed and uncompressed.

reference:
ISO Image - Wikipedia
What is the difference between an ISO file and an IMG file?