It's a digital alu_xor having two copies of ALUs, namely: alu1 and alu2. The outputs generated by both the ALUs are XORed to yield any discrepancies between the designs. Any difference in the result turns out to be conditions relating to faults.
BLOCK DIAGRAM
References
Refer to README for a quick start of how to use caravel_user_project
Refer to README for this sample project documentation.