What determines the file encoding for ${C:file.txt} = 'abc' ?

Question

What determines the file encoding for  
${C:file.txt} = 'abc'
I'm always getting ASCII as the encoding for file.txt after executing that assignment.

Answer 1

Thanks so much.   I'll keep looking for the MSFT doc on this.  I scanned Bruce Payette's book and did not find anything there.   
It turns out to be one of those "by rote" things you have to learn about PowerShell.
My concern about the lack of documentation is that MSFT might change the underlying code in the future to use Unicode and that might break some existing code.  If there was some MSFT provided documentation declaring ASCII as the intended encoding they
might provide plenty of warning if they do a switch in encoding.
I note also that if you try to write characters outside the ASCII set (see example below) that character substitution happens to find an ASCII character to use in place of the one outside the ASCII set.  In the example below a 'v' is substituted for
the '√' character:
${C:xo.txt} = '√'