Impersonal constructions are very far from being a binary system.
This last sentence is an example of the "voz pasiva refleja", and I find it very hard to explain, because it more or less overlaps with impersonal sentences, and this is hard even for native speakers.
While in the "voz pasiva" there is something/someone affected by something or someone else, in the "voz pasiva refleja" there is a "
sujeto paciente" who receives the action of the verb, and unlike the impersonal sentences (where we don't know anything about the subject), the verb agrees with the passive subject:
- Se vende
ropa.
- Se venden
zapatos.
- Se solicita
ayudante.
- Se solicitan
cocineros.
- No se permite
fumar.
- No se permiten
mascotas.
- El
secreto se sabrá pronto.
-
Todos los secretos se sabrán pronto.
- Las oraciones impersonales se usan cuando no conocemos quién es el sujeto, o es irrelevante.
[This is a sentence with "voz pasiva refleja"]
- Se sabe que es una voz pasiva refleja, cuando sabemos quién es el sujeto que recibe la acción del verbo.
[This is an impersonal sentence.] 
Keep asking questions, I know this is pretty confusing. I have to check notes and books on this topic all the time.
