As IBKR does not allow clients to simultaneously maintain both a long and short position in a given instrument, any order to sell (buy) will only result in a short (long) position to the extent the order quantity exceeds any existing long (short) position held and which would then be closed by execution of that order.
For example, if a client holds a long position of 200 shares of stock ABC and subsequently enters an order to sell 300 shares of ABC, that order, if executed, will result in the client holding a short position of 100 shares of stock ABC.
Also note that in order for a short sell order to be accepted, IBKR must be able to locate shares to borrow. Note that shares are not actually borrowed until settlement and this locate satisfies the regulatory obligation making a good-faith determination of availability. If IBKR is unable to borrow shares at settlement, your short position is subject to forced close out.