As described previously, the two main tasks of a ML workflow consist of training and inference. Training involves obtaining and preparing the data, defining the model, and then the real training. Once your model has achieved satisfactory results during training and is able to perform adequate predictions (including on data it hasn't seen before), your model can then be deployed and used for inference using data outside of the training set. Core ML provides a suite of tools to facilitate getting a trained model into iOS, one being the Python packaged released called Core ML Tools; it is used to take a model (consisting of the architecture and weights) from one of the many popular packages and exporting a .mlmodel file, which can then be imported into your Xcode project. 

Once imported, Xcode will generate an interface for the model, making it easily accessible via code you are familiar with. Finally, when you build your app, the model is further optimized and packaged up within your application. A summary of the process of generating the model is shown in the following diagram: 

The previous diagram illustrates the process of creating the .mlmodel;, either using an existing model from one of the supported frameworks, or by training it from scratch. Core ML Tools supports most of the frameworks, either internal or as third party plug-ins, including  Keras, turi, Caffe, scikit-learn, LibSVN, and XGBoost frameworks. Apple has also made this package open source and modular for easy adaption for other frameworks or by yourself. The process of importing the model is illustrated in this diagram:

In addition; there are frameworks with tighter integration with Core ML that handle generating the Core ML model such as Turi Create, IBM Watson Services for Core ML, and Create ML. 

Once the model is imported, as mentioned previously, Xcode generates an interface that wraps the model, model inputs, and outputs. You will get acquainted with these throughout the rest of this book, so we won't go into any further details here. 

In the previous diagrams we have seen the workflow of training and importing a model - let's now delve into the details of what this model is and what Core ML currently supports.