What Is A Comment?

A Comment is a free-text note carried inside the file. Nothing in the file depends on it: it is a blank line that whoever made the file could fill with absolutely anything.

Which files carry it, and where the text comes from

Almost every format has somewhere to put one. JPEG images have a dedicated comment marker, PNG stores comments as text blocks, MP3 files have a comment frame, MP4 and MOV files have a comment field, and documents, spreadsheets and PDFs have one among their document properties.

There are two ways it gets filled. Someone typed it, usually into the notes or comments box in a file's properties dialog or in an export screen. Or the program that wrote the file put something there itself. Software rarely uses this field for statistics about the file, since those have fields of their own. It uses it for whatever it wants to add: its own name, a leftover from a template, a licence line.

A file can hold several comments at once, and a report names each one after where it came from - an Apple QuickTime comment, an image comment, or a comment from the media log of an editing program such as Premiere. That prefix tells you which program or which part of the file wrote it. A comment can also be tagged with the language it is written in, so you may see a code such as en, de or fr beside it.

What the value looks like

There is no set format, because the field has no job to do. Things that commonly turn up:

• the branding of the program, app, website or online service used, for example Created with GIMP
• information about the creator, or a copyright line
• advertising, or a reminder about a licence or a trial version
• a project, client or job name typed by whoever handled the file
• nonsensical or placeholder text
• data that did not fit into any other field

An empty comment is just as common as a filled one.

Why it matters

Privacy first, because this is the field that leaks most often. Free text is where a real name, an email address, an internal job code, a customer's name or a colleague's note in a reused template ends up. All of it travels with the file when you send it, upload it or publish it, and nothing on screen ever displays it, so it goes unnoticed for years. If a file is going somewhere public, the Comment is one of the first rows to read, and one of the easiest to clear.

It is also the best clue to a file's history. A comment naming a program is often the plainest evidence of what a file was made with, which helps when you are trying to open an awkward file or work out why one file in a batch behaves differently from the rest.

It has no effect on quality or playback, and removing it changes the file size by no more than a few bytes.