Adding a Mac to iCloud

When signing on a new Mac to an existing iCloud account, we get the message:

"Your contacts, calendars, reminder, notes, and bookmarks will be uploaded and stored in iCloud so they can be accessed anywhere."

Will the current iCloud contents be purged and replaced by the Mac contents? Or would it be correct for the message to include:

"Your data will be merged with your existing iCloud contents."

Thanks!