Normally dimension means distance between two points. How is it described here?
The 150 indicates a dimension from the outside edge of a spline to the centre. The 300 indicates an overall dimension of both splines combined?
hopefully this helps answer your question. 2016 SOLIDWORKS Help - Spline Handles
The spline begins off being tangent to the handle. The number specifies the strength of the tangency relationship.
Generally I get the spline to look right, then add the 'dimensions' to define the curve so it doesn't go awol. The numbers are insignificant as long as the curve is correct (IMO).
How can this be explained? 500 and 150
The number that controls the handle is not a unit of measure. The higher you push it the longer it will stay with the direction of the handle. Unlike a unit of measure where there is a defined length/angle/radius, the number is doesn't fully define the curve as it is also controlled by the location of the next spline point, the direction and weight of it's handle. By assigning 500 to the lower left handle, you are giving it greater influence on the direction of the curve to the 300 (150 each way as pointed out by Dave) weighting put on the upper centre handle.
What is the purpose of this unit?
it allows you to define the curve. as in your original example, the 150 and the 300 (150 each way) and the 150 create a curve that has equal rates of change out of each point (as they're spaced symmetrically about the centre point). If a designer wants the curve to transition smoothly from one direction to another using equal weight achieves this.
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