The format specifier "C" (or Currency) is used to converts a number to a string that represents a currency amount. By default, the '$' currency symbol appears when converts a number to a string.

// By default, if you provide only "C", it displays the currency upto two decimal values
string.Format("{0:C}", 1456.12155)    // $1,456.12

CX -> X - indicates the decimal places, For example, if display 2 decimal places then use the "C2" currency format specifier.

// "C0" - No decimal values
string.Format("{0:C0}", 1456.12155)   // $1,456
// "C1" - Only one decimal value
string.Format("{0:C1}", 1456.12155)   // $1,456.1
// "C2" - Two decimal values
string.Format("{0:C2}", 1456.12155)   // $1,456.12
// "C3" - Three decimal values
string.Format("{0:C3}", 1456.12155)   // $1,456.122
// "C4" - Four decimal values
string.Format("{0:C4}", 1456.12155)   // $1,456.1216
// "C5" - Five decimal values
string.Format("{0:C5}", 1456.12155)   // $1,456.12155

Display different Currency Symbols:

CultureInfo instance needs to be passed to display the custom culture symbols.

// Default US Currency Format
string.Format(new CultureInfo("en-US"), "{0:C}", 1456.12155)      // $1,456.12
// Currency format for Denmark
string.Format(new CultureInfo("da-DK"), "{0:C}", 1456.12155)      // 1.456,12 kr.
// Currency format for Great Britain
string.Format(new CultureInfo("en-GB"), "{0:C}", 1456.12155)      // £1,456.12
// Currency format for Denmark fo Japan
string.Format(new CultureInfo("ja-JP"), "{0:C}", 1456.12155)      // ¥1,456

Table of countries and their Culture Info Codes: