Three types of DLLs supported by VC 6.0

NON-MFC Dlls (Non-MFC DLL) refers to the use of MFC's class library structure, directly written in C language DLL, its output function is generally standard C interface, can be written by non-MFC and MFC application calls. You can use the Win32 Application Wizard to create a non-MFC DLL project.

Regular Dlls (MFC Regular DLL), created by using the MFC DLL Wizard, is divided into static and dynamic links.

Extension Dlls (MFC extension DLL), created using the MFC DLL Wizard, always dynamically links MFC.

The following comes from msdn--to determine the type of DLL to use

If your DLL is to use MFC and will be used by MFC or non-MFC applications, you must generate a rule DLL that is dynamically linked to MFC or a regular DLL that is statically linked to MFC. In most cases, you might want to use a rule DLL that is dynamically linked to MFC because the file size of such a DLL is much smaller and using a shared version of MFC can save a lot of memory. If you are statically linked to MFC, the DLL will have a larger file size and may occupy additional memory because it loads its own copy of the private MFC library code.

Generating DLLs that dynamically link to MFC is faster than generating a DLL that is statically linked to MFC because the former does not need to link MFC itself. This is particularly true in debug versions where the linker must compress debug information. By linking to a DLL that already contains debugging information, only a small number of debugging information in the DLL will need to be compressed.

One disadvantage of dynamically linking to MFC is that shared DLL:MFCx0.DLL and MSVCRT must be published with DLLs. DLL (or a similar file). MFC DLLs can be easily republished, but you must still install DLLs in Setup. In addition the MSVCRT must be delivered. DLL that contains the C Run-time library to be used by both the program and the MFC DLL itself.

If the DLL is used only by MFC executables, you can choose to generate a rule DLL or an extension DLL. If your DLL implements a reusable class that derives from an existing MFC class, or if you need to pass MFC-derived objects between applications and DLLs, you must generate an extension DLL.

If your DLL is dynamically linked to MFC, you may want to republish the MFC DLL with a DLL. This structure is especially useful when you share a class library between multiple executables to conserve disk space and minimize memory usage.