An employment contract is usually entered for a definite period of time (D4). In many cases, there is the option of extending the contract and/or converting it into a permanent position after one year, subject to proven suitability, permanent staffing capacity and sufficient (influx of) students. If you respond to a vacancy, you can inquire about this with the contact person.
In addition, there is also work for a certain period due to replacement, due to temporary expansion of the activities of a department or for a defined project. This period is then explicitly mentioned in the vacancy: the contract is automatically terminated after this specific period.