In general, the Parclick reservation does not include a specifically allocated parking space. Some car parks do have specific spaces allocated to Parclick users, or the staff will tell you where you can leave your vehicle when you arrive.
For these car parks, you will find the appropriate instructions on the booking voucher that you will receive in the purchase confirmation email. Apart from these exceptions, you are free to choose the space you prefer, as long as it is not reserved for another purpose (e.g. disabled spaces or electric vehicle charging points).