Although you can enter AutoLISP code at the AutoCAD Command prompt or Visual LISP Console window prompt (Windows only), any functions you define are lost when you close the drawing or session they were created in.
AutoLISP source code can also be stored in files with a .mnl extension. A Menu AutoLISP (MNL) file contains custom functions and commands that are required for the elements defined in a customization (CUIx) file. A MNL file is loaded automatically when it has the same name as a customization (CUIx) file that is loaded into the AutoCAD-based product.
For example, on Windows, when acad.cuix is loaded, the file named acad.mnl is also loaded if it is found in one of the folders listed as part of the AutoCAD Support File Search Path. If a CUIx file does not have a corresponding MNL file, no error is displayed, the product just moves and loading other support files.