STAMINA (STA) is probably the most important stat in the game. Stamina represents your overall energy level. You use stamina for everything — from swinging a sword to casting a spell to walking around. Even defending against incoming attacks uses up stamina. Stamina is the green bar that you always see over characters and monsters, as well as down in the lower-left corner of the screen near your portrait.
Everyone starts with 40 stamina points but then it increases based on your Strength (+5 per point), Constitution (+3 per point) and Dexterity (+2 per point). It also increases by 5% per level. There are also skills that can raise your stamina permanently and positive/negative effects that can raise or lower your stamina temporarily.
Stamina is important for two reasons. First, as mentioned before, stamina is used for everything. Attacking with a melee weapon takes about 4 points of stamina per swing. Using a ranged weapon takes less, but still uses it. So does defending against attacks. Walking around also takes up stamina, although the amount used depends on the color of the grid squares as you move through them (green and yellow are 1 point per square and red uses 3). Using abilities can drain a significant amount of stamina. Generally, the higher the level ability, the more stamina it uses (that’s because it assumes you have more stamina as you increase in level).
If you run out of stamina, you enter a state referred to as 
EXHAUSTED. That is not good. Someone who is EXHAUSTED cannot attack for a few turns until they recover and movement will be severely limited. If you find yourself EXHAUSTED, the best thing to do is DEFEND each turn until you’ve replenished enough stamina. Also, if you don’t have the minimum stamina required for an ability you can’t use it.
The second reason why stamina is important is that when you drop below 50% of your maximum, you start taking both accuracy (ACC) and defense (DEF) penalties. At close to zero stamina, these penalties can be a 50% deduction. For example, if you normally have an ACC of 50, running on empty stamina will reduce that to 25! It is significant. If you’re at 25% of your maximum stamina, you’ll have a 25% deduction.
So how do you get your stamina back? That’s where 
STAMINA REGENERATION comes into play. Barring some negative affliction, your body will normally regenerate stamina at the start of each turn. The base amount is 2 plus that increases 0.5 per point of Fortitude over 8 and 5% per level. So, at level 1 a character with a Fortitude of 9 will have a stamina regeneration rate of 2.5 points per turn. You probably already see that if a melee attack takes 4, and you’re getting back 2.5, that’s a deficit of 1.5 per turn. Add in being attacked which will cost 1 and you’re actually losing about 2.5 points per turn. In battle, you will see your stamina go down. Using DEFEND will boost your stamina regeneration by 50%, making that 2.5 a 3.75 (more on DEFEND in the Battle Tactics section). There are also abilities that can replenish stamina as well as boost stamina regeneration for a period of time. Plus there are skills that can permanently boost the stat.
This is a good time to talk about MAXIMUM STAMINA (sometimes also referred to as RAW STAMINA). Down in the lower-left corner of the screen when you’re in a world, your maximum stamina is indicated by the white line that is on the green stamina bar. As you use stamina, a percentage of the stamina used reduces your maximum stamina (generally about 30%, 60% if you’re walking in those red square grids when moving). While stamina regeneration will allow you to recover stamina, it will not recover beyond your maximum stamina. So as that white line goes down over time in a battle or while exploring the world, your stamina is essentially capped lower and lower.
There are only two ways to recover maximum stamina. One is to camp and rest, which you cannot do in battle. That will recover it quickly and as the white line goes up, the green bar will follow. The other way is to use DEFEND. Using DEFEND will have two outcomes, depending on if you are hit or not while using it. If hit, your MAXIMUM STAMINA will regenerate by 50% of your normal stamina regeneration rate. If you are not hit, it will regenerate at the full amount (including the x150% bonus). So in our example of someone with a STAMINA REGENERATION rate of 2.5, if they use DEFEND and get attacked (hit or miss) on their next turn they will generate 2.5 points of STAMINA and 1.25 points of MAXIMUM STAMINA (2.5 x 50%). If no one attacked them, they would generate 3.75 points of STAMINA and MAXIMUM STAMINA (2.5 x 150% x 100%).
Old-School Mode: This more difficult option is available on the GAME page of the SETTINGS menu, and stamina regeneration is limited. To regenerate MAXIMUM STAMINA, you must successfully DEFEND, which means not getting hit. Doing that will replenish your maximum stamina by only 33% of your stamina regeneration rate (including the 50% bonus). This was the original design of the game, which some found too difficult, but it is available for those that wish the challenge of having to manage stamina.
One more factor will affect your stamina regeneration. Your carrying capacity is based on your Strength, and it is generally 40 
units plus 10 units for each Strength point over 8. However, this is your 
MAXIMUM CARRYING CAPACITY. Exceeding that weight 
will stop you in your tracks and you won’t be able to move at all until you ditch some of it or offload it to another character. But carrying excessive weight 
will affect your stamina regeneration. This kicks in at 50% of your maximum carrying capacity and the more you carry beyond that, the less stamina you will regenerate. At 90% of your carrying capacity you will stop regenerating stamina entirely. So keep an eye on this. You will see an icon over your character’s portrait that gives you an idea of how heavy your load is. And your own equipped gear (armor and weapons) counts toward this as well.
NOTE: You can automatically redistribute all loot amongst your entire party to even out the load, based on their individual carrying capacity. Use the 
BALANCE button on the INVENTORY screen to do this. Equipped gear is not moved around, but its weight is taken into consideration in the redistribution.