This use of 'above' is only found in writing. It means 'previously-stated'.
For example, I told you in the sentence above that you can't use it for white-boards. And the previous sentence could have read. "I told you in the above sentence ..." So you get to choose (as happens with 'below', and quite a few other adjectives).
Note that it's not always physically above; it could be lower on the previous page.
You can also use 'above-mentioned'. "The above-mentioned professor will teach the next class."
It can also be used as a noun-substitute if the referent is easily understood: "I hope the above answers your question."