Definition
Three segments (needs/plans/rates) cut at the SIOP plan period to determine what to do at a given point in time.
In detail
Time fences are three time segments on the SIOP planning horizon that define which elements can be adjusted at different points in time. They address the flexibility of plan changes, avoiding confusion from frequent adjustments. Unlike demand time fences, planning time fences, and rate time fences, it is the umbrella term, while the latter are specific segments. The typical output is a set of rules for plan adjustment authority, guiding when demand, mix, or capacity can be modified.
Example in practice
In the monthly SIOP meeting, the planner notices a demand decrease for a certain product. If it has entered the Firm segment (within the Demand Time Fence), materials, production, and capacity are all frozen and cannot be changed no matter what; new or reduced demand can only be pushed to the next period; the team instead adjusts the product mix in the planning fence (Mix segment) that has not yet entered the Firm segment, buying space for the next period's raw material orders.