SP Transfer is able to transfer various items between two Joomla! databases. You install SP Transfer on the destination site, and you transfer items from the source database.
In no case an item will overwrite an existing item with the same id during transfer if you choose it. There is also a restriction in Joomla! where you cannot have the same alias of an item under the same parent. Taking these facts into consideration there are two important options you have to decide before the transfer.
If there is duplicate id, whether to transfer the item with new id, replace existing items or not transfer at all.
The process of transfer is done in two steps. The first step is the actual process of copy and the second is to link parents in case of items with new ids.
The component keep history of what is already transferred. This is necessary to keep track in case you have a php timeout.
It is strongly recommended to backup your database before proceed with the transfer, just to be on the safe side.