Class definition outside module and package

Hi,

I'm new to SV verfication and hence appreciate your help on this.

Usually I've seen SV class definitions inside a module / package . I remember having read before that class definitions cannot exist outside these, and as such cannot be compiled standalone.

But strangely, I've seen IUS14.1 successfully compiling a  .sv file which contains only class definition. Do not see module/package constructs inside the file.

Am I missing something in my understanding? Thanks for your suggestions and help in advance.

Regards

Kishore