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 -

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