Prevent reordering of plan items in RTC plans
One answer
Plans are views on work items. They are basically a query for work items that can have filters. The way data is displayed can be configured, and edited, in the plan view. As far as I can tell, reordering only happens if you change data that requires reordering based on the selected plan view. So you might want to look into the display and sorting options for the plan view you are using.