The steps that you take to develop, compile, install, and use a UDF depend on whether the UDF issues operating system I/O calls and whether the UDF implements UDT functionality, such as transform or ordering. The meaning of I/O as it applies to UDFs is any operating system call that requires the operating system to retain a resource context, such as for open files or other operating system services. Such resource usage usually returns a handle to the caller that is used to access the resource and must be released when finished using it.