proof decision values

I built a document report that includes the Current Version » Proof Decision field.  I added a filter to include only those documents where this decision field is not blank.

In addition to the expected decision values--approved, approved with changes, changes required, pending--I’m also seeing a “-” value:
 

Does anyone know what exactly this “-” value reflects?  I don’t work a lot with proofs themselves (I’m just reporting on them), so I’m not sure what conditions might produce this value.

The API Explorer just lists the field as a non-groupable string, but it doesn’t specify possible values.

Thanks.