- Starfleet Officer Insignia -

Starfleet follows a simple system. All department are equal in authority.. and the only characteristic difference is the Rank of the individuals in each department, even if those Ranks are named differently.

Medical, Engineering, Science, Diplomatic, and Command, inherited the use of the original Earth Naval Rank system, where as the Marines have adopted the old Earth Army Ranks.  But needless to say, the Marine Ranks, whilst different in name and design, hold the same weight as there corresponding Naval Rank.

e.g. A Naval Cdr. has the same authority as the corresponding Marine Rank, in this case Lt. Colonel. As such a Naval Cdr. can give orders to a Marine Major, because the Naval rank of Cdr. is higher than the Marine Rank of Major. This system works both ways, a Marine Lt. Colonel can give orders to a Naval Lt. Cdr.

This is a good point to note that Naval ranks are used in all of the following departments;
Command
Medical
Engineering
Diplomatic
Security/Tactical
Science

Personnel who join the Intelligence section of Starfleet may do so only after they have graduated the Academy in one of the above fields. As a result that individual joins the intelligence service as experts in there chosen field, and thus retain there duty and dress uniform colour, e.g., a Medical officer would retain his/her teal colour. Only if a individual joins the administration side of the intelligence service will the uniform be changed. The new uniform colour worn would then be red. 

- Duty Uniform -

Comparison Of Naval & Marine Insignias & Rank
Naval Rank	Insignia	Abbreviation	Grade	Marine Rank	Insignia	Abbreviation
Fleet Admiral		FAdm.	A5	Field Marshal		FMar.
Admiral		Adm.	A4	General		Gen.
Vice-Admiral		VAdm.	A3	Lieutenant-General		Lt. Gen.
Rear-Admiral		RAdm.	A2	Major-General		MGen.
Commodore		Comdor.	A1	Brigadier-General		BGen.
Captain		Capt.	B6	Colonel		Col.
Commander		Cdr.	B5	Lieutenant-Colonel		Lt. Col.
Lieutenant-Commander		Lt. Cdr.	B4	Major		Maj.
Lieutenant		Lt.	B3	Marine-Captain		MCapt.
Lieutenant-Junior Grade		Lt. (jg).	B2	1st Lieutenant		1Lt.
Ensign		Ens.	B1	2nd Lieutenant		2Lt.
Cadet 4th Year		C4	T4	Cadet 4th Year		C4
Cadet 3rd Year		C3	T3	Cadet 3rd Year		C3
Cadet 2nd Year		C2	T2	Cadet 2nd Year		C2
Cadet 1st Year		C1	T1	Cadet 1st Year		C1

- Dress Uniform -

Comparison Of Naval & Marine Insignias & Rank
Naval Rank	Insignia	Abbreviation	Grade	Marine Rank	Insignia	Abbreviation
Fleet Admiral		FAdm.	A5	Field Marshal		FMar.
Admiral		Adm.	A4	General		Gen.
Vice-Admiral		VAdm.	A3	Lieutenant-General		Lt. Gen.
Rear-Admiral		RAdm.	A2	Major-General		MGen.
Commodore		Comdor.	A1	Brigadier-General		BGen.
Captain		Capt.	B6	Colonel		Col.
Commander		Cdr.	B5	Lieutenant-Colonel		Lt. Col.
Lieutenant-Commander		Lt. Cdr.	B4	Major		Maj.
Lieutenant		Lt.	B3	Marine-Captain		MCapt.
Lieutenant-Junior Grade		Lt. (jg).	B2	1st Lieutenant		1Lt.
Ensign		Ens.	B1	2nd Lieutenant		2Lt.
Cadet 4th Year		C4	T4	Cadet 4th Year		C4
Cadet 3rd Year		C3	T3	Cadet 3rd Year		C3
Cadet 2nd Year		C2	T2	Cadet 2nd Year		C2
Cadet 1st Year		C1	T1	Cadet 1st Year		C1