Hi there topsparks, and welcome to ECN.
I see from your profile that you're in London, which from the abbreviations you are quoting I take to be London, England and not London, Ontario.
The current references listed have specific meanings according to the British IEE, and the subscripts they use aren't particularly logical:
Ib = Design current of circuit
In = Rating of protective fuse or circuit breaker
It = Tabulated current rating of cable, i.e. the basic cable rating from the tables
Iz = Rating of cable in specific situation, i.e. after the application of correction factors for temperature, grouping, etc.
You can see some notes and worked examples relating to these at this link: http://www.tlc-direct.co.uk/Book/4.3.9.htm
The adoption of the symbol U for voltage is another of those amendments the IEE made to "harmonize" with common European standards. A subscript "o" is commonly used to refer to a nominal level, hence the use of Uo to refer to the nominal supply voltage.