Categories with unique members are particles like negation (`nicht'), infinitive marker (`zu') and superlative marker (`am').
The IMS-tagset includes another particle type for separable prefixes. It covers prefixes like `an', `aus', `ein', ...(mostly identical to prepositions), but also other (non-adverbial) prefixes like `rad', `statt' and `instand'.
``er kommt an/PTKVZ''
``er kauft ein/PTKVZ''
``er fährt rad/PTKVZ'' NB: not `Rad'
``es findet statt/PTKVZ''
``er hält das Haus instand/PTKVZ''