What does the following attribute do:
set_instance_assignment -name GLOBAL_SIGNAL OFF -to <signal_path>
Is there a reason to apply this attribute to a reset line?
A logic option that specifies whether the signal should be available throughout the device on the global routing paths. Global signals can be both pin- and logic-driven. Clock, output enable, register control, and memory control signals can be global signals. Turning on this option for a pin or a single-output logic function signal is equivalent to feeding the signal through a GLOBAL buffer. Turning off this option for a particular signal prevents any of the Auto Global options from using the signal as an automatic global signal.
Translation: this prevents the compiler from automatically putting the referenced signal on one of the global routing channels in the device even if it might improve performance of the signal. The global routing channels are dedicated routing resources used to distribute clocks and other high fan-out control signals, like a reset, but they are a limited resource in older devices (not as much in newer devices like Stratix 10 and Agilex).
You use the GLOBAL_SIGNAL assignment to either force a signal to use one of these resources (when set to ON) or prevent a global routing resource's usage (when set to OFF). You might want to set this to OFF for a signal if you have limited global routing resources or if the "insertion delay" penalty for getting the signal onto the global resource through one of the dedicated buffers adds too much delay to the signal, potentially causing timing issues.