Sync:
You can synchronize two different lists such as copy contact data from one list to another or delete the contact from 2nd lists in case of duplicate records or change any column's data if there is a mismatch.
Split:
It will always better to manage and handle smaller contact lists.
You can split a big list into smaller lists with this Split option.
E.g. if you have a list with 1 Million contacts, so you can split that list into 10 or 100 smaller lists (or any number) instead of handling one big list.
You can target each list individually and change the name or any details by updating it after the split.
