From a quick googling I think your Wahl-o-Mat question relates to the use of
befristeter Arbeitsvertrag. When I looked at some of the longer examples of usage at
www.linguee.de, it seems that fixed-term employment contract is the best translation. The problem in translation is I think specific to the laws of the two countries. Let me give four examples of fixed-term employment contracts in the US.
1) A college student is hired to work as a lifeguard at a swimming pool for the summer. The employer offers a contract for 10 weeks including hours per week and pay. There is an assumption that the student could be hired again each following summer as long as both agree.
2) A college student is hired for a summer internship. The employer has the student sign a contract stating the pay and that the job will run for 10 weeks. After the student graduates the employer may offer the student a permanent job if one is available and the performance during the internship was satisfactory to the employer.
3) A freelance writer is hired to produce documentation for new software for a company. The writer signs a contract which includes the rate of pay and the maximum number of hours that can be spent on the project. If the company continues to have the need and funding, the company can enter into new contracts with the writer.
4) A college professor with tenure receives an employment contract for one year of services at a state university. The contract provides the salary for the year and coincides with the fiscal year of the state. Since the professor has tenure the assumption is that a new contract must be provided each year as long as the state funds the position.
From what I read online, it seems that one issue in Germany is the use of a series of fixed term employment contracts instead of a permanent position. As you can see from my examples, fixed-term contracts are not unusual in the US and a series of contracts can be the best way to manage a position that is subject to funding from a third party.