That's a good question and to be honest I too was confused when I was developing it. Let me try to walk you through the process I had in my mind.

As you rightly pointed out, there is a global capital field under settings. This is supposed to be your total capital across all your asset classes, e.g. stocks, forex, commodities et al. Let's say this capital is 10L INR.

Now, you can create various portfolios for asset classes (e.g. Forex, Stocks), trade types (e.g. intraday, swing, positional) or a mix of both (e.g. Intraday Forex, Stocks Swing) and assign a capital you want to use for each of these portfolios. Let's say you want to use 2L capital for intraday stock trades and 3L for intraday forex trading and the rest for positional or swing stock trades. Ideally the total of all your portfolios capital should be same as the global capital, though there is no strict check in the system for that.

Having said that, you don't need to create that many portfolios. In fact, not many traders would actively trade all of these. If you are just doing intraday trading then you can set the capital of your intraday portfolio to be same as global capital setting.

Reason for having a global capital and individual portfolio capital is to help you analyse the trades and individual portfolio performance better. This way you can filter the portfolios and track your equity separately for each portfolio and also get a global view of how your total capital deployed is performing across portfolios. 

Hope this makes sense.