I am getting a scale helicopter with a DJI Naza-H installed.  No GPS.

What can expect from the ATTI mode?  Will it hold it's altitude?  Will it hold it's attitude?  Can someone explain the difference between Manual and ATTI mode

Hi jhawk1977. In case the GPS failed or absence the unit can function in ATTI mode holding his aLtitude (but not the aTtitude). That means that your model holds his aLtitude but in the case of strong wind it goes away remaining in that aLtitude. The difference between Manual and ATTI: In Manual you control completely your model including the aLtitude with the throttle/pitch stick. In ATTI the control is the same except you don't care about the aLtitude as soon as you have the stick centered because it is controlled by the barometric sensor inside the Naza.

Manual mode feels like that you are flying a Flybar heli without stabilizer.
Atti mode feel like you fly with a stabilizer.

In Manual mode, stick input = angular speed while in Atti it is velocity speed