Not aware that Poser has instances, below is based on those applications I've come across.

Instance - one master object which can be inserted into the scene many times. An insert being a transform consisting of  x,y,z position and 3D rotation. So 10 instances is one mesh plus 10 transforms.

Duplicate - copy of the a mesh each with its own insert. 10 duplicates in a scene mean 10 mesh objects plus 10 x postion/rotation values for each.

Instances are much more memory efficient than duplicates. Also if its a 3D modeller app, changing the master object effects all instances.