There are business rules that are capturing this, so SIC codes as of 2007 are 5 digits, so on an incorporation it must be 5 digits. We need to support 4 digits for companies filing an annual return still using a 4 digit code.
Nature of control again is captured by a business rule, the law does not permit you have more than 1 in the groupings of shares, for example the same person as a PSC cant say they hold 25-50% of shares and also say they hold 50-75% of shares, you need to give the total of shares they hold.
We will look to publish these business rules in the next few days to hopefully make it easier for developers