The basic difference is that the absolute existence has the meaning of cause, and relative existence has the meaning of consequence, which implies that there cannot be relative existence without absolute existence. Thus, relative existence can appear and disappear while absolute existence cannot. Absolute existence must exist without (external) cause because it originally exists – there is nothing else before it that could be its cause.
This implies that the original existence must exist itself by itself, that is to have the ability to be the causer of its own existence, keeping in mind the logic that implies in itself that nothing can exist without a causer, even that first, absolute cause, as the cause of all other causes.