Hydroboration on Limonene
The borane reagent in question, disiamylborane, is one of the many bulky hydroborating agents which preferentially add to less hindered double bonds.

In this case, the 1,1-disubstituted double bond outside the ring is more prone to hydroboration than the trisubstituted double bond within the ring, simply due to sterics.

There are many more examples of hydroborations with this reagent which are dictated by sterics: for further examples see its corresponding entry in the Encyclopedia of Reagents for Organic Synthesis. In all cases, disiamylborane is selective for terminal alkenes (i.e. either monosubstituted alkenes, or 1,1-disubstituted alkenes).