Swaps are used to temporarily substitute one employee on a list with another. This is general used when someone on an on-call has something to do for a set period of time then someone else on the list can take his place for that time frame. See the example situation below.
John is on call tonight but wants to see his son's ball game from 5:00 pm to 7:30 pm. John can call dispatch or his supervisor and have James take his place on the on-call list for that two and half hour time frame. At the designated start time of the swap, James will appear on the list in the same position as John. Once the swap has ended James will no longer appear on the list and John will be in his appropriate spot on the list.
Another way swaps can be used is when two lineman want change callout rotation for a short period of time within the same roster.
Dave is the second lineman on the list and is going to take Robert's position (tenth) on the list for 8 hours and Robert will take second position on the list for the same period of time.