NSEU,
Intruder is correct, it will return to alt...but its a function of when depressed. If you are in level flight, it stays, but in climb or descent, it must recover from such and maintain, and depending on your FMS manufacturer, the +/- value can make a difference.
I have worked with some that have a different accuracy on climb vs descent, and said accuracy can be 10's or 100's in the rounding factor.
Perhaps your application can help in the response.
Cheers!