Setting parameters at the API level enables the automatic assignment of the parameters to all resources and methods included in the API. Any parameter value you specify at the higher API level overrides the parameter value you set at the lower resource level or the lower method level if the parameter names are the same.

For example, say you have a header parameter called API Key that is used for the consuming an API.

This parameter is specific to the entire API and to the individual components - resources and methods directly below the API. Such a parameter can be defined as a parameter at the API level.