With Shares, it is part self-assessment and part what your trading patterns show. ie if you buy 10 stocks and sell one in 6 months later but still hold nine you are not really a "trader". You are more deemed an investor.OK, so what determines with a gain is income or capital? The length of time you hold a stock? <12months it's income, >12 months it's CG?
If you buy and sell daily that activity indicates you are a trader and doing it for income. The volume of trade and the intention at the time of purchase are important. You can be both - you may trade some shares but others you buy for long-term investment.
The tax system is all self-assessment so you can sort of pick and choose. But the pattern of trading itself almost decides it for you.