Slaves to Armok: God of Blood, Chapter II: Dwarf Fortress

The Manual: Soldier Preferences

How to Get Here

Press v from the main view and move to the desired unit. After that, press p followed by s.
You can also access the same information from the military screen, which you reach by pressing m, then select weapon preferences.

Description

Here you can designate which weapon and how many weapons a dwarf uses in battle. You can also set armor preferences here. A dwarf will not carry weapons unless they are activated for combat. If a dwarf is designated to carry a weapon, the mining profession will be turned off.
Heavily armored warriors move slower and make fewer attacks, but they are well-protected. Armor is generally a good thing, but it is expensive and time-consuming to construct complete sets. Dwarves will wear whatever is available up to their armor level.
Crossbows can be very effective, but this is only true in ranged combat. A marksdwarf will bludgeon with the crossbow if it comes to that, but a crossbow is not a good melee weapon.
The various melee weapons have different properties. Bludgeoning weapons do more raw damage but cause less bleeding and cause fewer internal injuries. Swords and axes balance these effects. Spears cause more internal injuries but less raw damage.
Because raw damage is based in large part on the size difference, using heavy damage non-piercing weapons like maces against giant creatures can be effective. At the same time, causing a critical internal injury to any creature can bring it down immediately, as well as slowly bleeding them out with many small wounds.
These considerations also apply to heavily-armored opponents. Spears in the hands of unskilled dwarves might not be as effective as a mace wielded by the same dwarf, but a highly skilled speardwarf can score more critical strikes.