It has long been a tradition in many museums in Europe, to allow artists to pay a fee and have access to "copy" works of art in the collection.
When copying a work of art, certain universal rules apply in order to protect the the original .
For example, it is not allowed for someone to make a copy with the exact dimensions of the original art work, or in some case using the same materials.
This prevents a copy being illegally passed off or sold fraudulently as the original.

In this image, the woman is making a drawing of this sculpture of an ancient River God. - neither the same dimensions nor material.
At one time in the history of this sculpture, the River God had water flowing from the vase in his hand.