The Instance object duplicates geometry of another object. Instance stores only the changes made to the original object. Therefore, it is a memory efficient way to create multiple copies of a same geometry.

Instance property gadget has the following options:

Ignore matrix - When set, the original object can be transformed (move, rotate, scale) after instance creation without changing the instances. If the option is cleared, instaces take into account all object transformations of the original object and its parent levels. Which way is better, depends on the case. Even when the option is set, it is still possible to modify all instances simply by performing the desired transformation to sub objects of the original object or to its actual points.

Render as instance - activates instancing in photorealistic rendering. The render engine does not convert instances to real objects but performs the instancing transformation during ray tracing. This typically slows down rendering by 20%-30%, but saves memory. For example, it is possible to model and render a forest by repeating thousands of instances of a dozen of basic tree shapes.

Applying render time instancing to simple basic shapes does not save memory, but actually can increase the memory use, because the transformation matrices require also some memory space.

[Note] Note
Render time instancing has some limitations. Currently render time instancing does not allow nested or scaled instances. Only translation and rotation are allowed. The program automatically disables render time instancing if these conditions are not set. Secondly, the applied geometric transformation may confuse complex VSL shaders which refer to 3D coordinate information.