This sets the Bit Error Rate threshold that triggers a Signal Degrade condition on this APS group's protected line, expressed not directly but as the exponent for a 10^-x threshold -- so entering 5 here configures a BER threshold of 10^-5 (one error per 100,000 bits), and entering 9 configures a much stricter 10^-9 threshold. Because a higher number here represents a stricter (lower) actual error rate threshold, an admin wanting the system to react to even mild line degradation would set this closer to 9, while an admin wanting to tolerate a noisier line before declaring degrade would set it closer to 5. For example, setting this to 6 means the line must exceed a 10^-6 bit error rate before a Signal Degrade condition (and potentially a protection switch, depending on priority) gets triggered.