Canīt understand Atmel datasheet (simple)

I am having a bit of a problem understanding a datasheet. It is a mega16 microprocessor from Atmel that has the spec. output to VOHmin=4.2V. At the
same page (p.292) it says that IOH=-20mA.
What I want to do, is to use this port to drive an led and an transistorīs bias. Should I calculate on the current (20 mA) or should I calculate on the output voltage? I am using a BSR14 transistor (NPN).. the rest must be calculated (resistors, minimum/maximum current running thru the led).
Very thankful for any help!!!
Kind regards Peter
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