A building specialization was supposed to be available at building tier level 3 and allow a player to choose a variant on the bonuses and abilities the garrison building provides.
Unfortunately, this functionality never really appeared in a publicly seen version of the Warlords of Draenor expansion. It may have been in some private alpha builds.
From "Take Charge of Your Garrison":
- For example, one Mine specialization might make your miners gather ore faster by the hour, while another will increase the chance that your workers strike a rare mining node.