You can use the following built-in functions with combination box objects.
Function  | 
Description  | 
bottom [of]  | 
Returns the line number of the last line in the combination box.  | 
top [of]  | 
Returns the line number of the first line in the combination box (which is always 1).  | 
textual line from  | 
Returns the text that appears on the specified line of the combination box. (See textual Object Inquiry Built-in Function in the Reference Guide.)  | 
selected line  | 
Returns the currently selected line in a for each selected line loop.  | 
selected line from  | 
Returns the line number of the line that the user has selected from the combination box.  | 
window  | 
Returns the line number of the first line  | 
yposition of  | 
displayed in the combination box.  |