At least for the time being, any save game files you create on a Nintendo Switch can't be transferred to another system.
The feature wasn't available in the hardware we got our hands on last week, and today's day-one update didn't introduce any new functionality that enables you to move save files from one system to another.
Save files are stored on the system itself, not an external storage card or physical game cards themselves. Nintendo said earlier this year that game cards are non-writable.
In the short term, this may not be a serious issue, but it's something we'll hopefully see added eventually.
It's no stretch to assume that such a feature could be planned for the future. We already know about a wide array of things that Nintendo plans to eventually add to Switch, including more ways to add friends, a premium online service, and Virtual Console. That's not to mention the potential for media streaming apps like Netflix, which have yet not been confirmed, though areas of the system's Settings menu suggest they could be eventually released.
For more on Switch, which launches tomorrow, March 3, check out our review and everything you need to know roundup.