The selection of a specicfication field is by far not a decision of life and death. Please keep in mind that when finishing this Master course you will be a “Materials Engineer from RWTH Aachen University” and you will apply for jobs with that diploma. Most probably you will not be asked “Which of the 7 specification fields did you choose?”. In the easiest case you may have a very clear view of what you will do after your studies, e. g. “I will work in a specific foundry company in my home country”. Consequently, you should immediately select the respective field. However, this is also the less common case. Mostly, students do not have a very clear idea of what they will do after their studies. Moreover, in most countries it is very unlikely that you will work in one company or one field for your entire working time nowadays . Thus, it does make sense to develop a rather broad field of expertise during your studies. Hence, it is recommended that you opt for a field which sounds interesting to you. In this case you will hopefully get good marks and you will get a (very) good diploma.
Please also be careful about hints and recommendations of other students saying that one field is more difficult or easier than another one. Both is incorrect as a general rule since you are all individuals with different talents, skills, and mostly different Bachelor degrees. Thus, it will always be an individual decision, which is exactly what this university is trying to convey to you: Self-responsibility!
Please also note that if you go for a very popular field you will be in a big group, whereas in less demanded fileds you will be automatically supported more individually.
Students starting in Winter semester in October do not have to decide on the specification field immediately upon enrollment since the first semester is identical for all fields. These students need to select a specification field at the start of the second semester, the latest, to get the courses assigned to their curriculum in RWTH online. Students starting in Summer semester do have to select their specification field immediately to get their courses assigned. For more details see here.
According to the rules students may change the specification once. There is no deadline for that. For a change of specification please use the respective form, available at the department´s webpage. Should the change of specification be carried out after the deadline for exam registration students will need to send a separate form to the examination board asking for a late registration. If this cannot be done in time their is still the option of manually registering for an exam at the institute and get the result acknowledged retroactively.
After the change all CPs obtained in subjects common to both specification fields will be transferred automatically. Subjects outside the curriculum of the new specification field will be tretaed as additional subjects. These appear on the diploma but are not affecting the average mark.