I see that any updates to scheduled script execution is not captured in the update set. What is the criteria to have changes captured? Can we manually configure the list of items to be and not to be captured in updates set.?
Tables with the attribute update_synch set to true are captured in update sets. This is the attribute set on the collection entry in sys_dictionary.
Scheduled script execution definitions (sysauto_script) should actually be captured in update sets, but the actual sys_trigger record which actually causes the scheduled script to be executed per the schedule is NOT update_synch'd, and that's by design. The sys_trigger table is modified heavily by the actual scheduler service (e.g. resetting next action on every execution, run once jobs created and destroyed for things like workflow timers)
Technically, you could add the update_synch attribute to a sys_dictionary collection entry to cause it to be captured by update sets, but that is highly ill-advised, unless you really know what you're doing.
You can manually add non-update-synch'd records to your update set ad-hoc by way of a script described on the servicenowguru website.