I've been working with and fooling around with the magnetic force field. I noticed the seed number slightly change depending on the size of the force, but I wasn't sure what it did or what it controlled. Knowing that would likely help!
More detail can be found here.
The seed input determines the noise pattern of the force. This makes the force uneven and more realistic, especially useful for wind simulations, but also when dealing with gravity, magnetism, etc. This is a random number, and the seed input is calculated into the noise result. An example is the seed you can type in when you open a new world in Minecraft. A different seed result in a different map. Unrelevant, I know, but I think this is a good example.
You can adjust the amount of force noise under the noise input. Reduce this to 0 for no noise, or bump it up to 10 for a very uneven noise, meaning e.g. a wind force will blow more at one spot than another.
Hope this helps.
Various data that is generated for things like simulations will use random numbers to get their results. In reality, we don't get true random numbers, we get pseudorandom numbers, which is a reproducible way of generating a sequence of numbers that resemble random numbers, with the random seed "defining" the sequence of numbers we get.
For simulations this is helpful as we can use the same random seed to always get the same sequence of numbers. This means every time we play the simulation we see the same result. When we don't like the result we can choose a different seed to get a different result.
The seed in the force field should not change when you alter the size. You should expect the seed to be different for every force field you add. If you don't like the way particles move with the force field you can try using a different seed.
The seed is actually part of the random number generator used to generate the noise. Changing the seed simply generates a new set of random numbers for the noise. But, the random number generator is pseudo random, so inputting the same seed again will yield the same sequence of numbers.