Many games treat weight and encumbrance as being equivalent terms. This is inaccurate, and can lend itself to abuses. Just to set the record straight, here are our definitions of weight and encumbrance.
Weight: How much mass a particular item has.
Encumbrance: How awkward a particular item is. This is based mainly on the volume of the item.
An empty cardboard box will have high encumbrance, but low weight. A bar of gold has low encumbrance, but weighs a lot. All rules dealing with encumbrance are optional, and are intended to supplement the rules dealing with weight.
An item of equipment has both a weight and an encumbrance. These must be added up separately for all equipment your character possesses, and must be under the maximum allowable for the character. Otherwise, something will usually have to be dropped. Extra weight can be carried through the use of periodic STR checks, and extra encumbrance can be carried through the use of periodic AGI checks (see below under Attribute Checks).
The maximum weight a character can carry is one-half of their STR, in kg. Thus a character with a 45 STR could carry 22.5kg. Note that this is different from the maximum weight the character can lift!
The maximum encumbrance a character can carry is one-half of their AGI. There really aren't any good units for measuring encumbrance; volume is a close approximation, but doesn't work for small but awkward items.
Mick has a STR of 60 and an AGI of 80, giving him a weight allowance of 30kg and an encumbrance allowance of 40. He is carrying equipment which has a total weight of 28kg and a total encumbrance of 35. If he finds an item which weighs 2kg and has an encumbrance of 6, he will have to make agility checks in order to carry it without dropping anything.
There are numerous items of equipment which are designed to carry other items (bags, packs, vehicles, etc.). In these cases, the item will have a maximum weight and encumbrance that it can contain. When an item is placed inside of another item, the encumbrance of the first item is ignored, and the weight is considered to be added to the weight of the enclosing item. So, an empty box will have the same encumbrance as the same box will when full, but will weigh much less.
If Mick had a backpack with at least 2kg and 6 encumbrance free, he could put the item he found into it. This would make the total weight he is carrying reach 30kg (his maximum), but his encumbrance would still only be 35.
Some items will have a different encumbrance value when worn than when carried. Most of these are items which are often worn, such as articles of clothing (but not armour) or backpacks.
In order to successfully carry more weight than would normally be allowed, a character must make a check against (STR - excess weight carried) every minute. If the roll fails, the load is dropped (check for breakage: see the section on Equipment Hit Points in the Equipment chapter), and must be collected.
Similarly, to carry more encumbrance than their maximum, a character must make a check against (AGI - excess encumbrance carried) every minute or drop their load.
Mick decides that he is going to try to carry the newly found item. He doesn't need to make STR checks, as he is only carrying 30kg, his maximum. However, he is now carrying 41(=35+6) encumbrance, which is 1(=41-40) more than allowed. Every minute, he must roll under 79(=80-1) to keep everything under control.
The amount of equipment a character is carrying can have adverse effects on their performance of duties.
Any skill which has Strength as the primary attribute will be affected by how much weight the character is carrying. Any skill which has Agility as the primary attribute will be affected by how much encumbrance the character is carrying. These include most weapon skills.
To determine the adjusted skill level, determine the percentage of the character's maximum allowance (weight or encumbrance as required) which is being used. Divide this by two and subtract the result from 100%. Multiply this by the character's normal skill level to find the effective skill level which should be used for the check.
Sasha is in a sword fight. Since Sword skill has Strength as the primary attribute, she must consider how much weight she is carrying. She has a weight allowance of 30kg, and is currently carrying 12kg. This means that 40(=12/30x100) percent of her weight allowance is used. Half of 40% is 20%, subtracted from 100% gives a result of 80%. This means that if she wants to make a sword attack, it will be at 80% of her normal Sword skill level.
If you decide to use this rule, make sure you have a calculator handy. It may be a useful abstraction to simply keep track of the normal weight and encumbrance a character carries, and ignore picking up and dropping small objects. If a character always carries the same equipment, it may be worthwhile to make a note of the adjusted skill levels for some of their commonly used skills.