It depends on the type of mission, but there are a few things you must have (as per US Army standards), spare magazines for your main weapon such as rifle/submachine gun, grenades (depending on the type of mission), batteries (basically all soldiers carry batteries these days, including spare batteries for communication equipment, lighting, etc.), flashlight (depending on the type of mission), mission information briefings (most of them are carried by the commanders but for special tasks or important information is everyone with), signal stick (lighting stick or smoke stick, depending on the type of mission, but the combat team generally always have one or two people with), multi-functional dagger (this some people are mounted on the body, a line of people hanging in the leg, some missions and not with) communication equipment (everyone has, but the level is not the same, and the function is not the same), first aid medicines (some of the special types of soldiers do not carry, think of communication soldiers do not carry), first aid medicines (this some special), first aid medicine (this is not the same, but the communication soldiers do not carry). Soldiers do not bring, think communication soldiers do not bring, but ordinary soldiers more or less will bring a little), personal belongings (allowed to bring some items that will not affect the safety of the mission, like photos, coins and what not, but depending on the mission) and so on.
Magazines are concentrated in the lower part of the undershirt, mostly the lower front (that's the part from the stomach to the chest), communication devices are usually in the left shoulder, daggers are usually in the left rear, batteries are usually in the back, torches are usually in the left arm or right chest strap, medication is mostly on the right side, and mission information is in the left chest strap.
These are just more cases, you go to find pictures to observe will know, how to correctly say can not be clear, and with the type of mission, the type of soldiers, countries, and personal habits related.