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This might be a very trivial question but as a beginner, I don't know the answer. I was exploring ? and after running a cell containing ?@, I got an interesting result:

enter image description here

However, I'm not sure about its exact interpretation. The answer contains two parts, a Global` part that shows the list of all the global variables that I have defined (or were created as a result of my code). But what is the System` part, and what does ?@do to get them?

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  1. The full form of ?@ is Information["@", LongForm -> False].
  2. LongForm -> False means use the short form of Information, which calls Names["@"] (or a utility that both Information and Names use) to find the symbols which it should retrieve.
  3. Names interprets "@" as

    match one or more characters, but not uppercase letters

So you are getting a short form display from Information for all the symbols that match the wildcard character "@" as described under the 3rd bullet point of Details and Options under Names

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  • $\begingroup$ Wow I was way over my head! Thanks. $\endgroup$ – MathX Mar 30 '16 at 2:39
  • $\begingroup$ If I may ask, what are those that look like Greek letters with dots above and below that show in the system part? $\endgroup$ – MathX Mar 30 '16 at 2:43
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    $\begingroup$ @MathX. See Formal Symbols under Letters and Letter-like Forms $\endgroup$ – m_goldberg Mar 30 '16 at 2:47
  • $\begingroup$ Thanks! This is a super useful link. $\endgroup$ – MathX Mar 30 '16 at 2:50
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    $\begingroup$ Yes. However, at that time, I was stuck by the special interpretation of ? by the front end, FullForm[Hold[?@]] won't work, one has to use prefix form FullForm@Hold@?@. $\endgroup$ – luyuwuli Mar 30 '16 at 6:16

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