This is commonly known as an API and it allows users to send and reveice Type-B messages from within their own programs. 

For which Sitatex versions are SDK's available?

There are SDK's available for all Sitatex versions:

Sitatex V3 - SDK V3

Sitatex V4 - SDK V4

Sitatex V5 - SDK V5

Sitatex V6 - SDK V6

Sitatex V7 - SDK V6

Programs written to utilize one version of the SDk are NOT compatible with the other versions and  will need a rewrite in case of a software upgrade!

What is the difference between an SDK and an API?

There is no difference apart from a different name. The name Software Development Kit is an equivalent for the name Application Programming Interface.