Do you mean DLL file type? It is designated by .dll at the end of filenames in DOS directory listings. Many programs use them for data storage, user settings, etc.
If you wish to write a routine that reads it properly, you need to know exactly how the file is organized, the order in which variables are stored, how data is formatted, etc.
If you wish to share a dll between two programs, then one of them needs to have a built-in option for importing or exporting data in the other program's native format.