The big difference is performance and the size of the resulting model. 'General' is the slowest, but it also works better when there's a chance that the topology of the 'instances' will not all be the same. 'Simple' is next in terms of better performance but there are some limitations. 'Identical' is the fastest but it's limited to when ALL of the 'instances' have the exact same topology as the parent feature, hence the name 'Identical'. For example, if your model consists of a flat plate with a perpendicular hole and you're going to make an 'instance' of these holes, then 'Identical' would be the best option.
Please note that starting with NX 8.0, 'Instance Feature' was replaced with 'Pattern Feature' which does not have these same options. There is still a 'Simple' for the easy cases and it is much faster and produces a smaller model (file size wise). But later in NX 9.0 we introduced an improved 'Pattern Face' which would have the absolute best performance and can often be used where you would have used the 'Identical' option with 'Instance Feature'.
I need some more clarification on this which i have mentioned below
In ugnx7.5 instace feature>circular pattern methods as you mentioned that in 'General' method topology of the 'instances' will not all be the same means Is there any chances for variation of parameters of the instanced features
No, that's not what I meant. The instances will all be linked to the same parametric feature or parent no matter what option you use, just that the TOPOLOGY, meaning the number and sizes of the faces and edges COULD be different depending on what the base model is like. For example, lets say that one or more of the instances in your bolt circle intersected with something like another hole. This would make the result TOPOLOGY different than the parent feature. 'Identical' would NEVER work, but 'General' would.