The format is using positional parameters as {0} {1} {2} embedded into the string.
Example:
lcName = "Rick" ? StringFormat("Hello {0}. The time is: {1}",lcName,DateTime())
Values embedded are embedded using the TRANSFORM () and converted to strings. If you require special formatting for values turn the values into strings and apply your own formatting:
lnPi = 3.14159265 ? StringFormat("Pi has a value of {0} when truncated.",TRANSFORM(lnPi,9.999))
Note that - for now - unlike C or C# there are a no formats applied since you can just as easily apply the formatting to values passed into the function. Values are simply rendered using the default TRANSFORM() encoding.
o.StringFormat