I'm not sure there is a precise definition for you, and if there is it's likely not well known enough to be applied accurately.
To me the difference is:
Unbalance describes a change, meaning it was balanced and it is now not currently balanced. It's primarily a verb, by doing it you cause something to be out of balance. But it is also a noun (unbalanced) and in that form is synonymous with imbalance.
Imbalance describes the state of being not in balance, it may, or may not be desirable. This is a noun, not a verb, though some may incorrectly use it that way.
The real difference is in the power handling. The actual power (and therefore current) provided by the transformer is determined by the load on the transformer.
The 1000 mA transformer can handle a load that consumes more power and also more current.
For the sake of simplicity, I'm going to assume a resistive load with no reactance, and therefore a power factor of 1. You will probably likely want to look up those terms if you're interested in learning more about transformers and AC power in general*.
For example,
If you have a 5W load (device you want to power) connected, it will draw 5W / 12 V = 417 mA (approximately). If you connected this load to either tranformer, it would draw 417 mA.
If you have, say a 10 W load, it would draw (or attempt to draw) 10 W / 12 V = 833 mA (approximately) - it would draw this if connected to the 1000 mA transformer. If you tried to connect this load to the 600 mA transformer then one of the following would occur:
The load would not function (because the voltage droops or sags too low to power the device)
The load would draw more current than the transformer can handle and possibly damage the transformer (depending on the transformer and load, could be immediate, or could slowly heat up and deteriorate over time)
The transformer fuses would "Blow" if installed and properly size for the transformer (to prevent the damage in #2)
This is a simplified explanation. Most transformers can actually handle a little more than what their rating states - so they won't always get damaged, but you should always stay within the manufacturer's specifications to ensure maximum life of the equipment.
*If the loads are reactive and have a pf other than 1, you cannot simply divide power by voltage to get current.
Best Answer
The withstand voltage is explained in the specification sheet for your part:
The withstand voltage is the maximum voltage where the manufacturer guarantees less than 1 mA leakage current and no damage to the part if it is applied for up to 5 s.
The "rated voltage" is 40% of the withstand voltage. Probably they intend the rated voltage to be the maximum nominal operating voltage for the part.