Originally Posted by
Bezalel
When staying within the same road groupset, short cage is for a double and medium cage is for a triple. In the change from 9 speed to 10 speed the capacity of the Tiagra groupset has been increased so when using a 30T cog it would be incompatible with almost all previous generation road derailleurs.
Not necessarily. In the 10 speed era Campagnolo made short (55mm, double with no cassette bigger than 12-25 or 13-26), medium (72.5mm, double with all cogs, triple through 12-25/13-26), and long (89mm) cage derailleurs.
When they introduced 11 speed triples in 2012 they added an 82mm medium+ derailleur alongside short and medium.