an example can be found on AMOSPro_Examples:Examples/H-1/Help_11.AMOS
This is used inside an IF..ENDIF or IF..ELSE..ENDIF structure. It performs a test, and if it's true, executes all the AMOS commands until the next ELSEIF, ELSE or ENDIF statement.
If A=1 Print "A=1" Else If A=2 Print "A=2" Else If A=3 Print "A=3" Else Print "Other values..." End If
Note that you CAN'T use an ELSEIF between the ELSE and ENDIF section.