Let's see if I can help you understand this part of the Spirometry Test, or confuse you more.
This test requires accurate measuring equipment.
This is your 'Forced Vital Capacity'. Essentially when you are sitting still and breathing slow and deep breaths, and suitably recovered from any exertions you may recently have done. Then take in a deep breath, as much as you can without undue strain, then breathe it out as much as you can, again without undue strain. The volume of that outward breath is your FVC. Usually this is expressed in litres.
This is your 'Forced Expiration Volume in 1 second'. Breathing as described above, take in the deep breath, as much as you can, then breathe out as fast as you can, for 1 or2 seconds, then continue to breathe out until you have to breathe in. The result of this is usually expressed as a percentage. The equipment takes two measurements while you are doing the outward breath. The first is the volume of air you expel in the first second of that breath. The second is total volume of air you breathe out in that breath. The figure you get for FEV(1) is then worked out by taking the volume of that first second's worth of air and expressing it as a percentage of the total volume of that breath.
Usually the test is done three times with sufficient time between tests for a good recovery. This is to make sure that you are giving a consistent performance so that a bad test can be identified and discounted.
A normal fit, healthy person would get a result of about 80%.
As far as I am aware, the current NICE guidelines say that less than 80% is mild
less than 50% is moderate
less than 30% is severe
As a stand alone test, this tends to show the loss of elasticity that a person has suffered, and gives an implication of the amount of scaring and other damage that your lungs have sustained. Where this type of test becomes important is when it is done twice. The first time before you have had any long acting bronchodilators in the day and no short acting within about six hours. After doing the first test, you would be asked to take your short acting bronchodilators, then take the test again after giving the drugs time to work. This gives an indication of how well your drugs work
There is more to a Spirometry test than I have mentioned here, but I think this covers the question asked.
If I am wrong in any of this, say so. I am not God, and do make occasional mistakes just to prove it.