• If you use the cffunction tag, your function can include CFML tags.
• If you write your function using CFScript, you cannot include CFML tags.
  
  You can use UDFs in your application pages just as you use standard ColdFusion functions. When you create a function for an algorithm or procedure that you use frequently, you can then use the function wherever you need the procedure, just as you would use a ColdFusion built-in function. For example, the following line calls the function MyFunct and passes it two arguments:

Recommended uses

• Data manipulation routines, such as a function to reverse an array
• String and date and time routines, such as a function to determine whether a string is a valid IP address
• Mathematical calculation routines, including standard trigonometric and statistical operations or calculating loan amortization
• Routines that call functions externally, for example using COM or CORBA, such as routines to determine the space available on a Windows file system drive
  
  Consider using UDFs in the following circumstances:

• You want to pass in arguments, process the results, and return a value. UDFs can return complex values, including structures that contain multiple simple values.
• You want to provide logical units, such as data manipulation functions.
• Your code must be recursive.
• You distribute your code to others.
  
  If you can create either a UDF or a custom CFML tag for a particular purpose, first consider creating a UDF because running it requires less system overhead than using a custom tag.

For more information