>That mean .. API is not a must in the application .. correct ?
>

There's no requirement that you use direct API calls to write a program. If you want to do some things not native to the programming language and available through an API call, then you need the API call. It's your decision when to use calls to an external service via an API call.

>is't the same case for DLL, ED?
>

In what respect? Using services of a C-satyle .DLL involves access to servces via an API call. VB and VFP do not create DLLs using the C-style calling interface, they use COM, which is a separate issue. There's no requirement that you use COM, unless you decide to use services offered through COM.