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esc + cc + esc gives a double-struck lower-case c. It is different, however, from the double-struck c given by esc + dsc + esc (the double stroke is curved in the former but vertical in the latter). Mathematica calls this character "\ConstantC". I can't find any information about its meaning or purpose. To add to the mystery, its Documentation Center page has a bullet point reading "Unicode: F7DA", but no such Unicode character exists.

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    $\begingroup$ It is used to represent arbitrary constants, e.g., DSolve[y'[x] == y[x], y, x] $\endgroup$
    – Bob Hanlon
    Commented Oct 30 at 18:21
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    $\begingroup$ If comments could be downvoted I would downvote both two comments above. $\endgroup$
    – azerbajdzan
    Commented Oct 30 at 18:26
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    $\begingroup$ @azerbajdzan I found Bob Hanlon's comment helpful because it gives a working example that quickly shows a "use case" for this symbol (that is, a case where Mathematica uses it). $\endgroup$
    – WillG
    Commented Oct 30 at 18:37
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    $\begingroup$ @WillG Actually Mathematica is not using it in his example but instead it uses C[1]. The fact that C[1] is displayed as \[ConstantC] is a different matter. $\endgroup$
    – azerbajdzan
    Commented Oct 30 at 18:42
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    $\begingroup$ @azerbajdzan Please be aware that your first comment was flagged as unfriendly. It would have been much better if you had said "I disagree with both of the comments above" and then explained your reason as you did in your last comment. $\endgroup$
    – C. E. ♦
    Commented Oct 30 at 22:13

2 Answers 2

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The \[ConstantC] character is used to format output expressions like C[1], such as those from DSolve. These outputs are actually wrapped in a TemplateBox however, so you can't use \[ConstantC] manually for input, unless you copy-paste one such output.

In[1]:= C[1]

Out[1]= C[1] (*formats with \[ConstantC] in the front-end*)

In[2]:= \[ConstantC][1] === C[1]

Out[2]= False
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To answer the second part of your question: U+F7DA is a perfectly valid Unicode character. However, it belongs to the so-called private use area, in which characters do not have a standard definition, but can be used by any application for its own purposes.

You can find this mentioned also in ToCharacterCode:

The Wolfram System defines various additional characters in private Unicode space, with character codes between 57344 and 63743.

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