Sync:
You can synchronize two different lists by copying contact data, deleting duplicate records, or changing any column's data in case of a mismatch.
Split:
It will always be better to manage and handle smaller contact lists.
You can split a big list into smaller lists with this Split option.
If you have a large list of 1 million contacts, it may be more manageable to split it into smaller lists of 10 or 100 contacts.
You can target each list individually and change the name or any details by updating it after the split.
