User blog comment:B214/Question and Answers/@comment-24.2.168.254-20140117001056/@comment-1359322-20140117020034

Usually a Sacred Gear is in it's normal state, and can enter into a Balance Breaker with the right requirement. In certain situation, the Balance Breaker might change into a subspecies Balance Breaker. A subspecies Balance Breaker usually takes the preference of the user. Like Cao Cao's Subspecies Balance Breaker is one that is Technique centred. Or Kiba's Sword of Betrayer which was done by fusing his SG + soul of his deceased comrades.

As to subspecies Sacred Gear, that isn't fully explained but it's due to the user's taste too. Like Siegfried wanted his Twice Critical to be able to help in wielding his Demon Swords, so it became a Dragon Arm while doubling his powers. And of course since the Sacred Gear is already a subspecies, the Balance Breaker will be one too.

But overall, a Subspecies Balance Breaker is more common than a Subspecies Sacred Gear. That's because the ability of the Sacred Gears. Most of the Sacred Gears' ability are kinda fixed after all, like Twilight Healing, or the Forbidden Balor View, Boosted Gear, Divine Dividing, Sword Birth, Blade Blacksmith.

Basically these Sacred Gear's abilities are high even in their base form, so it's impossible to convert the whole Sacred Gear into a subspecies, that's why a Subspecies Balance Breaker is more suited. Then for  Sacred Gears like the Twice Critical that simply doubles the attack of the user, it's simple to just change the whole SG into a subspecies.