Phrog30 is right.
I now realise that you want to know everything, however, in reality the things you need to know are the degree of accuracy for each of the components for example, load cell, amplifier & analogue conversion in the PLC. these should be obtainable in the spec sheets. Start by adding the accuracy of the load cell & amplifier this will give you some indication of the worst case error combined. This is not the true as on any particular weight it could be that the load cell is +0.01 gram out and the amplifier is -0.03 gram so the actual is less than the max error. by adding the stated accuracy that will give you the max possible error. The analogue card is a different matter, depending on if it is 12 bit or 16 bit conversion and the resolution for example a 12 bit A/D will only give you 2048 counts so if you were converting that to temperature of 0-200.0 degrees C then it is a little over 1 count to every 0.1 degree given that there is the analogue cards own conversion accuracy then you should be able to work out the total deviation possible & that is your accuracy (in a perfect world). However, there are many other factors, electromagnetic fields around the load cell, fixing type temperature all manner of things. Like I said earlier, if you are measuring 0-500g and the load cell gives out 10 microvolts per gram as the load cell is under load (your platform) of say 200g then your signal is not 0-5000 microvolts it's actually giving out 200 - 5200 microvolts or 200g to 5200g. so to choose a load cell to weigh 0-500 it needs to be capable to read 0-6000g as it is already under load. The 4-20ma will be 4ma at 200g & 20ma will be 5000g after all you will not sit what you are weighing directly on the load cell. I have been involved in many installations of weighing systems from pack weigher's measuring a a few grams up to tons. and also in the last 20 years done the 6 monthly calibration on vessels. I know the pitfalls of design & installation for example, one of the worst things you can do is use 4 load cells on a vessel, it is better to use 3 in a triangle, think of a chair with four legs and how it rocks, put it on three it's more stable. A lot depends on the design & the weight it is going to carry. I worked on a manufacturing plant where two of the vessels were on 3 load cells, however, the other was on a 4 load cell system. These were identical tanks & load cell mountings but the 4 leg would shift by a few kg so was not accurate enough perhaps down to engineering tolerances, however, they tried to shim it but it was not successful, They re-positioned 3 of the legs into a triangle & hey presto stable.
Just to re-iterate, concentrate on the accuracy it will work at and calibration.
I now realise that you want to know everything, however, in reality the things you need to know are the degree of accuracy for each of the components for example, load cell, amplifier & analogue conversion in the PLC. these should be obtainable in the spec sheets. Start by adding the accuracy of the load cell & amplifier this will give you some indication of the worst case error combined. This is not the true as on any particular weight it could be that the load cell is +0.01 gram out and the amplifier is -0.03 gram so the actual is less than the max error. by adding the stated accuracy that will give you the max possible error. The analogue card is a different matter, depending on if it is 12 bit or 16 bit conversion and the resolution for example a 12 bit A/D will only give you 2048 counts so if you were converting that to temperature of 0-200.0 degrees C then it is a little over 1 count to every 0.1 degree given that there is the analogue cards own conversion accuracy then you should be able to work out the total deviation possible & that is your accuracy (in a perfect world). However, there are many other factors, electromagnetic fields around the load cell, fixing type temperature all manner of things. Like I said earlier, if you are measuring 0-500g and the load cell gives out 10 microvolts per gram as the load cell is under load (your platform) of say 200g then your signal is not 0-5000 microvolts it's actually giving out 200 - 5200 microvolts or 200g to 5200g. so to choose a load cell to weigh 0-500 it needs to be capable to read 0-6000g as it is already under load. The 4-20ma will be 4ma at 200g & 20ma will be 5000g after all you will not sit what you are weighing directly on the load cell. I have been involved in many installations of weighing systems from pack weigher's measuring a a few grams up to tons. and also in the last 20 years done the 6 monthly calibration on vessels. I know the pitfalls of design & installation for example, one of the worst things you can do is use 4 load cells on a vessel, it is better to use 3 in a triangle, think of a chair with four legs and how it rocks, put it on three it's more stable. A lot depends on the design & the weight it is going to carry. I worked on a manufacturing plant where two of the vessels were on 3 load cells, however, the other was on a 4 load cell system. These were identical tanks & load cell mountings but the 4 leg would shift by a few kg so was not accurate enough perhaps down to engineering tolerances, however, they tried to shim it but it was not successful, They re-positioned 3 of the legs into a triangle & hey presto stable.
Just to re-iterate, concentrate on the accuracy it will work at and calibration.