Create curve in modelica

Curve is not a function. It is a class that is looked up and it is instantiated with the name FScurve.

There is no top-level Curve available as default in Modelica. So I would say the example in your book either defined Curve somewhere or it simply assumes that a class Curve exists.

Due to the way Name lookup is performed in Modelica, Curve could be in the same package as the model Spring or anywhere in the path up to the top level.

Example 1: Two curves, one local
- Curve
- Mechanics
  |- Spring
  |- Curve    // Fscurve refers to this class

Example 2: One curve at top-level
- Curve       // Fscurve refers to this class
- Mechanics
  |- Spring
  |- Foo    

Example 3: Two curves, none local
- Curve       
- Modelica
  |- Curve     // Fscurve refers to this class
  |- Mechanics
     |- Spring
     |- Foo    

The benefit of that mechanism is, that you can use short class names when you are working inside a package and you don't have to type the full class path (but you always can). If you have e.g. an electrical package

- Electrical
  |- Ground
  |- Resistor
  |- Inductor
  |- RL_Circuit

the example RL_Circuit could look like that:

model RL_Circuit
  Ground ground;
  Resistor R1;
  Electrical.Resistor R2;  // refers to the same class as R1, but using its full name
  Inductor L1;
equation
  connect ...
end RL_Circuit;