Since the expression occurs in a sentence that mentions PC's, I suspect it's a specialized jargon within the field of computers, based on the expressions 'top down' and 'bottom up'. These have to do with the direction of development: downward starting from the top vs. upward starting from the bottom, where 'top' refers to the most general view of a project and 'bottom' refers to the detailed view.
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mitsuwao23Still I am unable to come up with a good translation of itMe neither, if that's any consolation to you. You may have to leave it out of your translation. Just say "additional" or some form of that word.