A Decorative is an object that is used for, well decorative purposes. Here is a list of stuff the decortive is in Mapmaker. Note Decoratives only appear in TS3.

Military: A helmet on an end of a gun standing vertical. (note: you can't take the gun) Lab:A machine similar looking to a microwave on a stand, reminiscent of a Star Trek replicator.

Horror: a stuffed bear. Egyptian: a golden bird egyptian god statue. VR: a blue and red version of the lab object.