Onmitsukidō

Special Forces Also called the Secret Mobile Unit/Corps or Covert Ops, the Onmitsukidō (隠密機動; lit. secret tactics, called the Stealth Force in the English dubbed version).

The Special Forces was originally a separate branch of Soul Societies army, traditionally tied to the Shihōin family. When the Commander-in-Chief would gain a title within the Gotei 13 the Special Forces would become attached to that division as well. The Special Forces is generally commanded by a member of the noble family Shihōin, though the last member to hold this position was Yoruichi Shihōin before she departed Soul Society around 100 years before the main story line. Since that time, the Special Forces seem to be highly merged with the Gotei 13's 2nd Division.

Special Forces Ranks
The head of the special forces is called. The leaders of each of the divisions are ranked as. Although given that the special forces is now tied with the Gotei 13's 2nd Division each Corps Commander is given a Seated Officers position in said division. The leader of the Special Forces (Commander-in-chief aka. 2nd Division Captain) generally also commands its top division, which is the Executive Militia and it is known that the 3rd Seat Officer is the Corps Commander of the Detention Unit. It may be said that the order of the five divisions also goes in order of the top five officers of the 2nd division. (Unlike the Gotei 13 Captains, the Commander-in-chief has several bodyguards, though its current Commander-in-chief Soifon has never been seen with any).

Current
Commander-in-Chief
 * Soifon inherited both the title Commander-in-chief of the Special Forces and Captain of the 2nd Division after the departure of Yoruichi.

Former
Former Commander-in-Chief & Former Corps Commander of the Executive Militia
 * Yoruichi Shihōin

Former Corps Commander of the Detention Unit
 * Kisuke Urahara

Special Forces Divisions
There are five divisions within the Special Forces, from which the first one has top authority, and the fifth has the least one. The five divisions in order of authority are:

1.) Executive Militia (刑軍, keigun)

2.) Patrol Corps (警邏隊, keiratai): their apparent job is to patrol the various lands and dimensions to combat dangerous hollows and other enemies of Soul Society.

3.) Detention Unit: is responsible for the imprisonment and supervision of all criminals within the Seireitei. They also had another duty known as Special Detention which was related to those who joined the Gotei 13 but who were judged to be a danger to their fellow Shinigami. They would investigate, apprehend, and keep them sealed away from the rest of Soul Society within the special underground detention facility aka. the Maggots' Nest. It should be noted that those few kept in the Maggots' Nest have not actually committed any crime but have been identified by the laws of Seireitei as "Dangerous Elements". This division acts somewhat similar to prison and mental dentition center.

4.) ?: The forth division's name or function is unknown.

5.) Reversal Counter Force or Correctional Force (裏挺隊, riteitai).[8]

Special Forces Uniforms
Although the general special forces uniform appears to be the standard Shinigami uniform, the Executive Militia uniform is different from the Gotei 13. Overall, it resembles a ninja uniform. It has no visible white undergarments and the sash is dark as opposed to white. A headpiece covers the bottom part of the face and sometimes the top, but not the eyes; they wear no sandals, but long jika-tabi, and tight bands are on the legs and arms to keep the clothes from moving too much, which both are presumably for silence of movement. In the anime, members of the Executive Militia also wear their head covering on the top, not just the bottom and wear a black sash instead of a white one.

The leader of the militia wears the same uniform, except that the shoulders and back are exposed. The reason for the empty space is the eventual usage of, an advanced technique that combines hand to hand combat and kidō, causing high spiritual pressure to accumulate at the shoulders and back, blasting away the fabric at said places.

The Correctional Force uniform is radically different. The soldiers wear a white uniform, a long piece of headgear, and a backpack. 

Notes and references
Bleach chapter -106