At an acceptable level, the answer would be yes to all three questions. An API is by definition something that requires an application to use it. The app may be a ready one, such as AmiBroker that is discussed here, or a custom one that you develop. There may be multiple APIs -- one for order placement and another to fetch data. Also, there may be multiple versions of the API with different levels of functionality offered.
As to the specifics related to the different APIs that are offered by Omnesys such as name / number / price, it is best answered by the provider. That should be your first port of call. However, they generally redirect to your broker, or a specific third-party, which is okay so long as you get the info you want. We would love to know what you find out.