The most common type of 3D printing technique is called stereo-lithography which uses a vat (container) of photo-polymer resin. During the process of manufacturing the liquid polymer is exposed to light, where the UV laser light draws the cross section of material by hardening it, layer by layer and repeats the same process to forma complete 3D structure or a prototype.
The designing process is called bottom up, i.e. pulling the object out of resin. This creates some space for the uncured resin at the bottom of the container which then leaves some scope for the formation of the next layer of the prototype.