The OBJECT Is styled text command returns True when the "Multi-style" option is checked for the object(s) designated by the object and * parameters.
The "Multi-style" option lets you use rich test areas including individual style variations. For more information, refer to Multi-style (Rich text area) in the Design Reference manual.
Multi-style objects can be managed by programming using the commands of the "Styled Text" theme.
Passing the optional * parameter indicates that the object parameter is an object name (string). If you do not pass this parameter, it indicates that the object parameter is a field or variable. In this case, you pass a field or variable reference instead of a string (field or variable object only).
A form contains a field represented by two different objects; one of the objects has the "Multi-style" property checked, and the other one does not. You can write:
$Style:=OBJECT Is styled text(*;"Styled_text")
$Style:=OBJECT Is styled text(*;"Plain_text")