DLL are not data files. DLL are dynamically loaded library (hence DLL) that are executable code files. The code within the DLL files can be used to read and write data to files of various types. I would recommend binary data files for numbers. The code in DLL's are loaded dynamically when Windows needs it.
DLL's can be purposly written for PTC Prime. See help.
About Custom Functions
With custom functions, you can extend the functionality of PTC Mathcad Prime by writing your own customized functions. Your custom functions behave like PTC Mathcad Prime built-in functions. A custom function carries the Function label and can have many of the same features as PTC Mathcad Prime built-in functions, such as customized error messages, interruption, and exception handling in case of overflow and divide by zero. Custom functions are allowed to pass complex scalars, complex matrices, and string variable types. You can group a set of custom functions in a Dynamic Link Library (DLL). A DLL that groups several custom functions is called an extension.
