Complex sentence: Non-functional requirements, which are unverifiable at the software level, must still be kept as a documentation of customer intent; however they may be traced to process requirements that are determined to be a practical way of meeting them.
Split into simpler sentences: Non-functional requirements, which are unverifiable at the software level, must still be kept as a documentation of customer intent. However, they may be traced to process requirements that are determined to be a practical way of meeting them.