The acceleration curve is a linear graph measuring how quickly a space craft accelerates, which in turn is used to rate maximum sublight speeds.
The practically ubiquitous Ion vacuum drive has a maximum exhaust velocity of around seventy-percent of the speed of light (PSL). Thus, every ship using this engine has a theoretical maximum speed of 70PSL. However, the length of time it takes a ship, as well as the distance traveled, to reach that velocity, may make it impractical to cruise at that speed.
Many factors effect the acceleration curve: the engine itself(and it's specific impulse and exhaust velocity), the weight of the ship, it's role, and safety margins. Keeping in mind that a spacecraft has exert signficant force both when accelerating, decelerating, and turning, all of which factors in to the curve.
According to the engine's specific impulse, it may or may not have enough delta-V to reach 70PSL within an allotted time-frame. The acceleration curve is used to calculate the various speed ratings. The exact velocity differs greatly from ship-to-ship, and changes as well according to role. A carrier is, for example, expected to have a very different acceleration curve than a fighter.
The Acceleration Curve sets the ship's maximum recommend speed rating. There are two types of ratings: a hard maximum and an experience maximum. A ship rated at "40PSL" should never attempt to travel faster than forty percent of the speed of light. A ship with a rating of "40PSL+" is rated to travel at faster speeds according to the pilot or captain's discretion. Typically, only millitary spacecraft are given a "+" rating, though some civilian vessels(such as rescue ships) will occasionally receive it.