It depends on the type of mission, but there are a few essential items (based on US military standards): spare magazines for primary weapons such as rifles/submachine guns; grenades (depending on mission type); batteries (almost all soldiers now carry batteries, including spare batteries for communication equipment, lighting equipment, etc.); flashlights (depending on mission type); mission briefing (mostly carried by the commander, but for special missions or important information, everyone carries one); signal flares (either a flare or a smoke flare, depending on mission type, but usually one or two members of a combat squad carry one); multi-tool (some carry this on their person, others carry it on their lap, some missions don't); communication equipment (everyone has this, but the functions vary depending on rank); first aid supplies (some special forces don't carry this, like communications personnel, but ordinary soldiers carry some); personal belongings (some items that won't affect mission safety are allowed, like photos, coins, etc., but this depends on the mission), etc.
Magazines are concentrated in the lower part of the vest, mostly in the lower front (from the stomach to the chest). Communication devices are usually on the left shoulder, daggers are usually on the left rear, batteries are usually on the back, flashlights are usually on the left arm or in the right chest strap, medications are mainly on the right side, and mission information is in the left chest strap.
