Describing spread can be complicated. Below is an extracted quote from Dabalta:

What is spread in SC :
Spread is a random bullets deviation from where bullets spawn in °. So from the barrel in hipfire and near the scope's reticle when you ADS.
Instead of the bullets coming straight out of the "barrel", they will have a random angle between 0° and the actual spread value.
For example, if the actual spread value is 1.2°, the bullets can go 1° up, 0.5° left or 0° up/down, 0° right/left if you lucky. 
You can see the spread in this video : https://youtu.be/bPth2n6WMHE?t=205.
It's the green cone which increases when he shoots.

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How it works :
Spread section : spread of the weapon/fire mode during hipfire.

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ADS Spread section : spread of the weapon/fire mode when ADSing.

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ADS Spread Modifier (Multiplier) section : multiply Spread section by these and you get the ADS Spread section.

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Charge Spread Modifier (Multiplier) section : how much charge fire mode multiply the spread of it's fire mode. 

For example, Custodian has a burst fire mode that can be charged. The burst fire mode has it's own spread value and to know the spread value when this fire mode is charged, you have to multiply the spread of the burst mode by Charge Spread Modifier (Multiplier). But all this is already calculated in the spreadsheet.

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Spread data :
- Min : minimum bullet deviation that the weapon/fire mode have. The first shot will have a deviation between 0° and Spread Min.
- Max : maximum bullet deviation that the weapon/fire mode can have.
The maximum bullet deviation of the weapon/fire mode will be between 0° and Spread Max.
- First Attack : by how much the first shot will increase the spread value.
- Attack : by how much shots after the first shot will increase the spread value.
- Decay : by how much spread value is decreased per second.
- Decay/Shot (calculated by me) : by how much spread value is decreased between each shot if you shoot as fast as the weapon can.
-/Shot (calculated by me) : by how much spread value increases per shot if you shoot as fast as the weapon can after the first shot.
- /s (calculated by me) : by how much the spread increases per second if you shoot as fast as the weapon can after the first shot.

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So to resume spread, your first bullet will deviate between 0° and Spread Min and will increase the spread value by Spread First Attack. Then the spread value will decrease by Spread Decay/Shot between your first and second shot and your second shot will increase the spread value by Spread Attack. And it will continue up to Spread Max.

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For example I'll take the Burst 3 fire mode of the Gallant during ADS :
The first bullet will deviate between 0° and 0° (Spread Min) and the spread value will increase by 0.61° (Spread First Attack). 

Between the first and second shot the spread value will decrease by 0.25° (Spread Decay/Shot) so your second bullet will deviate between 0° and 0.36° (0°+0.61°-0.25°|Spread Min+Spread First Attack-Spread Decay/Shot) and the spread value will increase by 0.52° (Spread Attack).
Then again between the second and third shot the spread value will decrease by 0.25° (Spread Decay/Shot) so your third bullet will deviate between 0° and 0.6° (Spread Max) (0.36°+0.52°-0.25°=0.63° but Spread Max is 0.6° | last spread value+Spread Attack-Spread Decay/Shot)

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Something to know is that spread is bugged since many game version. Spread is a square instead of a round and the vertical or horizontal spread can become equal to zero and be added to the other spread "direction" so instead of being a square, it can be a line.

https://youtu.be/uYpB4FGeizI
https://youtu.be/sNnQXBGNuPQ

So if we take for example the third bullet of the Gallant Burst 3 fire mode :
instead of having a vertical and horizontal spread of 0.6°, the vertical spread may become 0° and the horizontal spread will be 1.2°.

https://issue-council.robertsspaceindustries.com/projects/STAR-CITIZEN/issues/STARC-16788