REALbasic's functionality may be extended through "plug-ins", which are small code libraries written in C or C++. These may include code for 68k Macs, PowerPC Macs, or Windows x86 machines. Plug-ins can define new functions, classes, or controls, which (once installed) appear in REALbasic just like the built-in ones. When the REALbasic user builds a stand-alone application, any plug-in code or resources used are also compiled into that application, so that end users (using just the compiled app) do not need to have the plug-ins on their systems.
Useful references relating to REALbasic plug-ins include:
The first two numbers indicate the major and minor numbers of REALbasic for which the SDK was written. So in SDK 2.1.1a4, "2.1" means that the SDK was written for REALbasic 2.1.
The third number (and any alpha/beta number) indicate the revision of the SDK, with respect to the version of REALbasic for which it was written. So in our example of SDK 2.1.1a4, this is revision 1, alpha 4 of the SDK written for REALbasic 2.1.
In addition, there is now an API Reference which documents each function in the Application Programmer's Interface -- the set of functions which let your code communicate with the REALbasic engine.
If you've already got a classic MacOS plug-in, and want to make it also work under Mac OS X or Windows, see Targeting New Platforms.
(There is currently no way known to use a source-level debugger on x86 plug-in code.)