From Ulož.to help
What is a free slot
Slot is a download thread, one slot = one synchronous download
Server has exact amount of free slots, which you can use to downloading files for free. When this amount is reached, you can downloading only through VIP account, which has a lot of other advantages (reconnection in case of interruption, unlimited number of synchronous downloads, unlimited speed).
Limit of synchronous downloads for free users is for whole server (the limit is not for individual user or file, but it is the maximal possible amount of all synchronous downloads).
We don't mention the concrete number of free slots, because amount of free slots is variable.