Inside Macintosh: AppleScript Language Guide / Part 2 - AppleScript Language Reference

Chapter 8 - Handlers / Using Subroutines

Scope of Subroutine Calls in Tell Statements

If you need to call a subroutine from within a Tell statement, you must use the reserved words of me or my to indicate that the subroutine is part of the script--not a command that should be sent to the object of the Tell statement.

For example, the minimumValue subroutine call in the following Tell statement is unsuccessful, because AppleScript sends the minimumValue command to the Scriptable Text Editor. (You get an error message saying
that the Scriptable Text Editor does not understand the minimumValue command.)

tell application "Scriptable Text Editor"   minimumValue(12, 400)
   copy result as string to word 15 of front document
end tell
(* result: the subroutine call is unsuccessful because
AppleScript sends the minimumValue command to the 
Scriptable Text Editor *)

If you use the words of me in the subroutine call, as shown in the following Tell statement, the subroutine call is successful, because AppleScript knows that the subroutine is part of the script.

tell application "Scriptable Text Editor"   minimumValue(12, 400) of me
   copy result as string to word 15 of front document
end tell
(* result: the subroutine call is successful because the 
words "of me" tell AppleScript that the minimumValue
command is part of the script *)

The word my before the subroutine call is a synonym for the words of me after the subroutine call. For example, the following two subroutine calls are equivalent:

minimumValue(12, 400) of me
my minimumValue(12, 400)