This static method of the System.String class is very, very useful.

It permits you to insert values into a string, like in this sample:

cString := String.Format( "Today is {0}", DateTime.Now:ToShortDateString() )

.

You can also control the formatting:

cString := String.Format( "It is now {0:d} at {0:t}", DateTime.Now )

For more details please see the Microsoft documentation: Get started with the String.Format method

Here are a few samples:

String.Format( "'{0,10}'", 100 ) -> '       100'
String.Format( "'{0,-10}'", 100 ) -> '100       '
String.Format( "{0:0000000}", 1234 ) -> 0001234
String.Format( "{0:0000}", 1234 ) -> 1234
String.Format( "'{0,10}'", 1234 ) -> '      1234'
String.Format( "'{0,4}'", 1234 ) -> '1234'
nValue:ToString( "10" ) -> 11234 
nValue:ToString( "4" ) -> 4
String.Format( "{0:yyyy.MM.dd HH:mm:ss}", DateTime.Now ) -> 2019.07.30 11:49:36

nDecimal := 123456789.987654321m
String.Format( "{0}", nDecimal )  -> 123456789,987654321
nDecimal:ToString( "############.##" ) -> 123456789,99
nDecimal:ToString( "############.#######" ) -> 123456789,9876543
nDecimal:ToString( "#.##" ) -> 123456789,99

nValue := 123
nValue:ToString( "0000" ) ) -> 0123
nValue:ToString( "####" ) ) -> 123
String.Format( "'{0,4}'", 123 ) -> ' 123'