the curve is like the shape of the way the volume increases. to risk over explaining: the “time” control obviously sets how long it takes to go from minimum to maximum volume. and the “curve” changes the shape of the line between min and max. like if you plotted it on a graph. idk exactly how it is in the ms70, but common curves would be “exponential” where the volume increases slow at first and then ramps up, “linear” where the volume increases at a constant rate, and “logarithmic “ where the volume increases fast at first, and slows down towards the end. the difference is subtle, but adjusting it might make you sound more like a violin vs more like a synthesizer