Some methdologies develop each subsystem fully then integrate them in a big bang at the end. Agile practitioners develop whatever parts of each subsystem are needed to support the features in the current iteration. If you think of the subsystems as layers, the vertical slice is the part of each layer that is being developed in the current iteration. The features should be prioritized in order of importance, so each iteration is working on the next most important feature or features. The idea behind all this is to always have some sort of working application.
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Some methdologies develop each subsystem fully then integrate them in a big bang at the end. Agile practitioners develop whatever parts of each subsystem are needed to support the features in the current iteration. If you think of the subsystems as layers, the vertical slice is the part of each layer that is being developed in the current iteration. The features should be prioritized in order of importance, so each iteration is working on the next most important feature or features. The idea behind all this is to always have some sort of working application.
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