The platoon sergeant is the second command of the platoon. A person taking the platoon sergeant role should be one who has a lot of experience in both leading fireteams and squads. The platoon sergeant is a jack of all trades and will be used as the commander sees fit. If you wish to be a platoon sergeant you should aim to be familiar with all the documents on this tactics wiki.
The platoon sergeant's job is to pro-actively work towards the plan of the platoon commander. Pro-actively doing anything to minimize the workload of the platoon commander and ensure the plan is carried as smoothly as possible. The platoon sergeant is also in charge of equipment and causalities and should always understand the capability of the platoon.
The platoon sergeant can be used in a vast number of ways. Possible uses of platoon sergeant:
Having the platoon sergeant solo/near the commander and having them ready to run any errands is extremely useful. This provides a great deal of flexibility depending on what arises during the mission. By default they pro-actively attempt to help in the execution of the plan as they fit. Unless directed by the commander. Some things they might be handy for:
After every objective is reached the platoon sergeant should pro-actively collect ammo reports and casualty reports. They should then redistribute ammo and potentially soldiers as necessary without the need for the platoon commander to be involved. This provides the platoon commander with time to plan for the next objective without needing to deal with any logistical issues.
The obvious thing a platoon sergeant can be used for is to command a collection of elements (squads/attachments/vehicles). In mechanized elements (with 3-5 armoured vehicles) it is quite typical for the platoon sergeant to be either placed in charge of the vehicles or all the dismount squads.
Be cautious with assigning all the armour to a platoon sergeant unless it is your intention for the armour to stick together. In platoon sized missions it can be far more effective to spread the armour alongside the infantry to provide more effective fire support. Thus having armour directly under the command or liaising with the nearest squad leader is typically more effective.
A common mistake is to put the platoon sergeant in charge of one squad even if there are a few extra attachments. A squad leader is typically competent enough to do this on their own. A platoon sergeant should be in charge of at least 2 squads or 3-4 attachments (Armoured vehicles).