“Combine” merges two parameters into one and updates all the DataPages where it is used and replaces the parameter that is being eliminated with the new one. Use it when two parameters are supposed to have the same value and are for the same purpose.
“Split” is the reverse of Combine. When you look at the Occurrences of a parameter and you want one of the occurrences to have a different value, you can Split it and set its value. The DataPage is updated automatically.