Final answer:
A 'danger' disc is used to indicate that a system is partially out of service as a safety warning to personnel.
Step-by-step explanation:
The correct word to fill in the blank in the given statement is 'danger.' Thus, the complete statement should read: 'A danger disc of 8-9 inches shall be placed on connections to indicate the system is partially out of service.' This is a safety protocol in engineering and maintenance fields to visually communicate to personnel that a system or part of it is not fully operational and to warn of a potential hazard. Such indicators are crucial for maintaining safety standards and for alerting workers to exercise caution around the tagged equipment.