What is an XML processing instruction, and why is there so little information about them?

I couldn't google a good explanation about them. W3 only has a little paragraph about them. Can anybody explain them in a clear form please? What are they used for? What does the "Processing instructions (PIs) allow documents to contain instructions for applications" definition from W3 mean? Are they still useful or long forgotten and substituted by something else?

Solution

Can anybody explain them in a clear form please?

That paragraph is clear. They provide a way to give an application an instruction.

It isn't detailed, but that is because any specifics depend on the particular PI.

What are they used for?

Describing the encoding and version of XML
Pointing to a stylesheet
Embedding executable code
Many other things

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