{identifier} in template files, what does it mean?

[super_serge]

Numerous times throughout the template .html files, identifiers enclosed in braces appear. Can anyone please tell me what function these have, how they are processed, and how I can find out where they are defined. Thanks, Serge.
I'm not sure whether to put this here or in the questions forum, but I want to change the values of some of them and am wondering how to do that.

[~HG~]

What is the name of the template you are referring to with the term in it?
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~HG~

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[super_serge]

Most of the files have them, for example, open overall_header.html and look at lines 14-15: