What is the use of VBA/API?
I know programming in Visual basic 6 but don't know it's application in CAD Softwares?
An API is an application programming interface - it allows code to interact (usually) directly with functions and libraries that are built into a piece of software..
VBA is visual basic for applications. VBA is a subset of visual basic (not visual studio) - VB is usually coded in 'VISUAL BASIC' language.
Uses of VBA APIs are infinite - and many types of applications have a VBA interface or scripting option of some type (not always an 'API')
The programming interface also does not always have to be VB or a MS language - it can be Java, Python, C++, etc. Some APIs are 'language agnostic', and others want to see very specific usage.
In CAD VBA can be used to automate tasks, augment built in features, provide new functionality, re-use included functionality in a new context, and so on.
ive written VBA scripts to create almost finished sections of sheet metal insulation for nuclear reactors - I've also written a VBA script that uses RS-232 to listen to a WII nunchuk controller.. More common uses are to automate printing, to do things like rev up design files, renaming features, creating features, automating FEA, etc. and so on.
Automation etc.. It really is a big topic! Go read Wikipedia about VBA and API.. Both articles will help you.