If you are a limited company you will have issued shares to the company's owner/s. This category represents the value of those shares, for example you may have issued 10,000 shares all at £0.01 each. Therefore resulting in the company having £100 in share capital.
The actual value of the shares paid for by the company's owners.
When you incorporate a business, the limited company will issue shares to its owners at a certain price per share. For example you could set up your company with 1,000,000 shares all worth a penny. That would mean you have £10,000 worth of share capital and that amount would sit in this category.
What's not included
Current Year Earnings and Retained Earnings both don't affect the amount of share capital that the company has on offer. The only thing that would affect it is if the company were to offer more shares when doing a further raise. This would increase the value of share capital the company has on offer, if it were raising at a higher valuation.