Action name: SetSegmentRegisterDefaultRelevant only for processors with the segment registers.
You can specify a default value of a segment register for the current segment. When you change the default value, IDA will reanalyze the segment, taking the default value when it cannot determine the actual value of the register. This takes time, so do not be surprised if references are not corrected immediately.
To specify a value other than the default value of a segment register, you can use change segment register value command.
See also Edit|Segments submenu. See also How to enter segment value.