Final answer:
The 1st Sub flag signifies that the submarine is making turns for 5 knots.
Step-by-step explanation:
The 1st Sub flag refers to a submarine that is making turns for 5 knots. In naval operations, flags are used to communicate different messages between ships. Each flag has a specific meaning assigned to it. The 1st Sub flag specifically signifies that the submarine is making turns for a speed of 5 knots. This flag is used to indicate the speed at which the submarine is moving and is important for maintaining safe navigation and coordination with other vessels in the area.