As their site states, this is something that you should ask them beforehand, since it is dependent in the individual situation.
The applicant's contract as a research assistant (wissenschaftlicher Mitarbeiter) has been extended by one year to continue working towards a PhD.
Since the contract has changed since the last application, taking the new contract with you would be advised.
BUS Rheinland-Pfalz - Elektronischer Aufenthaltstitel (eAT) Verlängerung
Welche Unterlagen werden benötigt?
Reisepass und weitere Unterlagen - je nachdem, welche Verfahren vorausgegangen sind. Da der Einzelfall entscheidend ist, sollten die erforderlichen Unterlagen vorher bei der zuständigen Stelleangefragt werden.
Which documents are required?
Passport and other documents - depending on the previous procedure. Since the individual case is decisive, the necessary documents should be requested from the responsible office beforehand.