ISIS and Twain are scanner languages that allow the user’s scanning software to communicate to the scanner.  Which driver to use will be determined by the software that you select.  Some software packages are designed to use the ISIS driver, some software packages are designed to use the Twain driver; and some other software packages are designed to use both.  Both drivers can be install on the host PC at the same time without conflicting with each other.  Please contact the manufacturer of the software you are using to determine which driver to install.