CBT stands for Computer Based Test.
RRB exams like NTPC, Group D and ALP are conducted as computer-based tests at exam centres.
CBT-1 is the first screening stage, and CBT-2 is the second stage for qualified candidates.
The syllabus for CBT-1 and CBT-2 is similar, but CBT-2 has more questions and higher difficulty.
Wrong answers carry negative marking in the CBT.
The exact pattern is given in the official RRB notification.