Hi Gaurav,
At the time of forfeiture, the company cancels the issued shares for non-payment of certain money called. Whatever amount has been received on such shares is a gain (need not to be refunded) for the company, accordingly it is credited to Share Forfeiture A/c.
Now, at the time of partial re-issue of shares, the balance of share forfeiture on re-issued shares is transferred to capital reserve and share forfeiture of unissued shares (forfeited) is added to the subscribed capital. The portion of share forfeiture belonging to unissued shares has been received on account of share capital, therefore it is added to subscribed capital.