Defending against threats is a basic reason for military groups. While a settlement may have fortifications manned by this military group, what we’re looking at here are their ways to protect themselves.
Regardless of genre, a military group typically mandates a minimum requirement for armor. When was the last time we saw a knight wearing only leather? It could happen if he’s on light guard duty deep inside a well-fortified city that hasn’t been attacked in a decade. Even then, his superiors would have approved this before it happens unless he’s headed for disciplinary action. This is sort of detail brings our world to life.
We don’t have to invent armor types in fantasy. Unless adding magical properties to it, everything we need probably already exists: leather (studded or not), chain mail, plate mail, plate armor, and variations on these. Understanding the differences helps us decide what they wear, including cost, how cumbersome they are to don, fight in, or wear for extended periods.
For example, chainmail is heavy but can be donned by one person, so a character prone to solo travel might prefer this. By contrast, full plate armor generally requires help, which is one reason knights have squires. Plate mail, which can be light and easier to put on, is a compromise. Since we’re talking military groups, it can be surmised that they often work with others, but perhaps not. Form an opinion about their likelihood of working alone as part of deciding that they use plate armor. Similarly, if they go many successive hours on duty, perhaps chainmail isn’t their usual garb due to the fatigue it causes.
We may want to decide that those of a given rank have superior armor; after all, commanders are more valuable. Consider the chart as an example, using army ranks to demonstrate.
Figure 3 Armor and Rank
If you’re wondering why a captain would wear the heavier plate armor and the more senior generals would only wear plate mail (arguably less protection), captains are the highest rank that’s in the field and expected to fight. Plate armor is unwieldly and impractical, so there’s little reason to believe a general would wear it; plate mail is easier to deal with while also conveying supremacy due to an appearance like that of plate armor. Throw in an elegant cloak or sash and the generals can look more regal.
In SF, ranged weapons like guns (regardless of what sort of projectile they fire) reduce the need for the sorts of armor we expect in fantasy. Even so, body armor does exist. The mundane Kevlar and similar materials found in real life can be used, but we can invent armor that deflects or lessens the damage of our invented SF weapons. World builders should think about protective clothing so that everyone isn’t only wearing their uniform. Some range weapons will work on the principle of a projectile, meaning the force of it striking causes damage. Other weapons like a laser can cause burning wounds, so heat resistant armor is more effective. Another beamlike weapon may emit radiation, whether a known kind or something of our invention. Either way, just like real world clothing offers limited protection against radiation, we can decide that some protection is incorporated into body armor but that it has limits.
Our military members might be trained in hand-to-hand combat with and without weapons. Is boxing part of training or do they only brawl? Our sense of their refinement and dignity can help us decide. The kinds of missions they go on can, too. Covert work often leads to close fighting – so close that swords (and longer) aren’t feasible. If such work is reserved for special forces like Marines, then perhaps the special skills are, too, with only basic fist fighting practiced elsewhere. Never being disarmed is unlikely.
There are various forms of martial arts on Earth and we can invent hybrids or our own. We’ll want a new name regardless of how much we borrow from one either in philosophy or form. It’s okay to describe one in such a way that people recognize what it is but realize we’ve changed the name, because most people won’t identify it anyway and the names of many are specific to Asian countries that don’t exist in our fictional world.