Configurator Format(x,y) function's format strings

The Configurator Tech Ref guide has the following entry for the Format() function

string.Format(format, object1, …)
String Format( x, y, …) Converts a value of any data type into a character value using format string.

But fails to mention what the format strings are. Nothing in the ICE 3.1 Tools Guide either.

Where can I find them?

Or… How do I convert a 1 to 3 digit integer to a string, and keep the leading zeros?

Its C #