Does the UDID of the iOS simulators change for every session?

I am trying to run my iOS script using UDID but script is failing though session is getting created.
What I observed from Appium logs is that the UDID is getting changed during run time.
When I give the deviceName instead of UDID as ‘iPhone Simulator’, almost every time, it is launching iPhone 6 simulator only though I do not specify the specific device. So, I tried to give the UDID but throwing the error as below
[simctl] Error: simctl error running ‘install’: An error was encountered processing the command (domain=NSMachErrorDomain, code=-308):
[simctl] The operation couldn’t be completed. (Mach error -308 - (ipc/mig) server died)

Please help how to get this issue resolved.