Hey Pineapple, Welcome to the forum.
Think of it like a bus schedule. The bus has 60 minutes to go through its entire route. The schedule has been carefully laid out so that the bus does not arrive too early or too late at any stop. Sometimes it means the bus waits a little longer at a stop just to be sure it isn't too early at the next stop. The riders expect that bus to be at their stop at an exact time each and every day. And they expect it to drop them off at a specific time.
You've added a new stop on the route. The schedule then must be re-calculated to make sure everyone still gets picked up and dropped off at the proper times.
As Alfredo mentions, RSNetWorx is the tool that recalculates the schedule. When scheduling, you can choose to "Optimize and rewrite the schedule" which throws out the existing schedule and starts over. This is the most efficient method but requires all network PLCs to switch to Program mode. Or you can choose to "Merge changes into existing schedule" which is less efficient but only requires the affected PLC to switch to Program mode. Which sounds bad, but it would have had to switch to Program mode anyway to download your changes. Merge is usually the preferred option since it doesn't impact other network devices.
So with Optimize, we might adjust everyone's pickup and drop off times to get the most efficient bus route. With Merge we would need to fit that new requested stop into the existing schedule without affecting any of the other stops on the route.
In recent years you have been able to get by without scheduling as a sort of temporary workaround. But that really wasn't ideal. We could go into all the details of scheduled vs unscheduled bandwidth, and the NUT, but that is in the category of TLDR.
OG