Metadata: UserComment, ImageDescription, Description; what's their purpose?

Question

$ exiftool test.jpeg | grep 'This is'
Image Description               : This is the 'ImageDescription'.
User Comment                    : This is the 'UserComment'.
Description                     : This is the 'Description'.

$ exif test.jpeg | grep 'This is' 
Image Description   |This is the 'ImageDescription'.
User Comment        |This is the 'UserComment'.

$ exiv2 test.jpeg | grep 'This is' 
Exif comment    : This is the 'UserComment'.

Three different commands show three different subsets of these tags.

Just to add more confusion, when the image is uploaded to Google Photos only the "Description" is shown.

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What is the intended purpose of these (and perhaps other) similar descriptions, and in practice how do people typically use them?

Note that I'm not asking about the technical details of differences in how they are stored as meta data; that's easy to look up. I want to know why one would decide to use one rather than the others, or to be safe should one always fill in all of them with the same information?

Answer 1

Here you can see the short description of specific EXIF tags. More detailed information is on official CIPA web site (200 pages PDF)

About usage - this is different story, you can use most of ASCII based tags for any text you want. If this make sense? I do not know. You do not need to fill all the tags with info. You may define your criteria what to fill by the purpose of the image, by the software you use to search/select image(s) by tag/tag value, etc.

Let me give you a example. I fill in Lightroom "Caption" tag just because GettyImages ignore "Title". And for me is more sample to copy and paste the content of "Caption" in "Title" (this is in Library, Metadata)

Answer 2

It has been said, great thing about standards is there are so many to choose from.

The standard use of those fields depends on which (if any) meta-data standard is relevant. There are a lot of common standards which could be applicable.

Without a reference to a standard, there is not a standard use. The schema establishes the right and wrong ways of using the fields.