Chief of staff

The chief of staff is an official, officer or general who is directly responsible for the work of the main body of military (force) management - the headquarters - in a combat situation (wartime) and supervises its training, education and daily activities in peacetime. The chiefs of staff in the armed forces of all countries are usually in the ranks from battalion (division) and above.[Schubert, F.]
The chief of staff, as a rule, is the first deputy commander (commander) and is personally responsible for:
Support of constant combat and mobilization readiness of troops (forces) and management bodies;
Planning of combat operations (operations), studying and processing the data of the situation, which are necessary for decision-making by the commander (commander);
Organization of preparation and provision of combat operations;
Ensuring reliable communication with subordinates and cooperating troops (forces);
Uninterrupted operation of control points, their movement, security, and defense.[Lavery, Brian]

Sources:

Schubert, F. (1994). Mobilization in World War II. Washington: U.S. Army Center of Military History.

Lavery, Brian (1989). Nelson's Navy: The Ships, Men, and Organization. Annapolis: Naval Institute Press.