Diversity is simply realizing that you probably will not turn on everything you have at the same time (they never met my daughter) Diversity shows up most in residential calculations. In industrial settings it would not be unusual to compute everything at 100% of it's rating plus the applicable overhead where necessary since it is on when the plant is operating. Usually we have engineers doing that here on the mainland if this is a big operation.
It is a little complicated at first. It is usually best to start by looking at a few. Basically you start with the square footage at 3va per sq/ft, apply a correction factor to that and then start adding in the fixed equipment. There are actually 2 methods you can use.
There are examples in the back of the NEC book but you can also find some work sheets and even a couple Excel templates on the net.