Hi,On B1900c, I'd like to know the difference between "ALT" and "ALT SEL". Thank you for your help.Regards,tcc0928

The same would apply to all aircraft with this autopilot function:ALT is an altitude hold function. That is the altitude you are currently at will be held by the autopilot.ALT SEL is a function allowing you to preselect an altitude, to which the aircraft will climb according to a vertical speed (V/S) or IAS hold function...Hope this helps...Andrew

Yes, Andrew is correct.If you look at the PMDG 1900C main panel, directly to the right of the top COM radio is an altitude set knob. You "select" your alitude here.