Categories are used for implicit Intents. So, If your Activity can be started by an implicit Intent when no other specific category is assigned to activity, activity's Intent filter should include this category. (even if you have other categories in the Intent filter). If you are sure that your activity must be called with any other Category, don't use the Default.
Setting Category to Default doesn't mean that this Activity will be used by default when your app launches. The Activity just says to system that " Oh I could be started, even if the starter Intent's category is set to Nothing at all ! "
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