Vector3int16 is a variant of the Vector3 datatype. Vector3int16, as its name implies, is a type whose coordinates are stored as 16 bit signed integers, which means that the coordinates of a Vector3int16 must be in the range of -32767 to 32767. Additionally, the Vector3int16 datatype is stripped down in terms of functionality. It is exclusively used for creating a Region3int16, which is used in some of the terrain object's methods.
Source: ROBLOX Wiki - http://wiki.roblox.com/index.php?title=Vector3int16