Each OpenFisca Country Package web API comes with a set of endpoints including an OpenAPI specification on the /spec route.
You can check out the demonstration swagger documentation to see how the endpoints work.
The OpenFisca web API can be used to:
access information about the parameters (e.g. /parameters), the variables (e.g. /variables), and entities (e.g. /entities) of the Country Package,
run simulations (e.g. /calculate) on a specific situation. To describe a situation, learn more about the web API inputs and outputs,
analyse a simulation calculation (e.g. /trace) on a specific situation.