When IDA encounters an instruction which changes a segment register, it creates a segment register change point. So, mostly change points are maintained by IDA itself. IDA assumes that the segment registers do not change their values between change points. If you find out that IDA failed to locate a segment register change, or if you want to change a register value, you can create a change point using Change Segment Register command. You can change the value of a segment register using Set default segment register value command too.
IDA classifies the change points. In the list of the change points, you can see the following postfixes after the register values:
a (auto) - Created by IDA. May be changed by IDA afterwards.
u (by user) - Created by user. IDA will not change it.
IDA generates the appropriate 'assume' instructions for the change points if it was not disabled by the corresponding command.