api is normally just interfaces, utilities and simple classes that have no transitive dependencies.
The service is where all of the implementations of the things defined in the api are, this is where transitive dependencies will also be found but, because they are not part of the api, the dependencies are limited to the service.
web is the UI portion of your app. it will have portlet code, action handlers, jsp pages, verification logic for input data but otherwise little business logic. Here you might have one web module with multiple portlets or you could make separate web modules with individual portlets.
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