A scripting addition (or osax) is a compiled code fragment, typically written in a language such as C, that extends the AppleScript language (see "Scripting Addition" in Chapter 3). A scripting addition has a dictionary, but it can't be targeted, and doesn't need to be; the commands that it implements are present as if built into AppleScript itself (see Chapter 20).
A scripting addition can define events (commands), records (pseudo-classes), and coercions. Generally speaking, a scripting addition can fulfill two purposes:
The default Tiger installation includes one scripting addition that adds to AppleScript's powersthe StandardAdditions osax. Many of the commands that it implements are so fundamental that this book treats them as part of the core languagefor example, display dialog, which is used in examples throughout the book, and the very important load script and store script commands (see Chapter 8). (The other installed scripting addition, Digital Hub Scripting, implements no functionality; it defines events, which are discussed in Chapter 26.)
Writing a scripting addition is beyond the scope of this book. If you're interested, Appendix C lists some resources that may prove helpful.
For some tips about speed when accessing a scripting addition command, see "Scripting Additions" in Chapter 22.