I'm in the process of rereading a book about WWI and was focusing on the general staff structure that Black Jack Pershing put together:
Modern United States military usage
Professional development: specialized staff training for lower, mid-level and senior officers is made available and necessary for both future assignments and promotion. The concept behind the duality of an officer's role in field/staff assignments is to provide field experience that can be later used at staff level. The following are designations used in the United States Armed Forces:
1 is for Administration/Personnel.
2 is for Security/Intelligence.
3 is for Operations/Training.
4 is for Supply/Logistics.
5 is for Public Affairs (Division and above).
6 is for Communications, and Information Systems (Corps and above).
7 is for Joint Operations (Corps and above).