The flexible job shop scheduling problem (FJSSP) is a generalization and extension of the classical job shop scheduling problem (JSSP) in which — prior to the sequencing of operations — an assignment of operations to machines is necessary. In this technical report, we are examining common instances for different specifications of the FJSSP. Furthermore, a new FJSSP specification is introduced, where similar machines are pooled to work centers, and the first and last work center are obligatory. For this practice-oriented problem specification new test instances are presented.