Some skills "branch" into specializations, which means after mastering 
the basic skill you must take a specialization of that skill in order to 
train further. Most skills and specializations on the skillweb are 
restricted according to archetype and more guild. Once you master the 
basic skill, you cannot train in that skill further, so must choose a 
specialization and train in that specialization.

For example, only the mage archetype can take the Elementalism skill and 
only mages in the Aquamancer Guild can take the Aquamancy specialization. 
After mastering elementalism, the mage cannot train in it further 
and instead would choose and then train in Aquamancy.

Some archetypes and guilds allow a choice among several specializations.  
For example, any who follow the knight archetype can choose to specialize 
in either the Blademaster or Bonecrusher branch of Knighthood.