Animations are sequences of frames. Since the definition of a scene for a single frame can be quite large, and since the differences for successive frames tend to be small, mental ray is based on a scene database that remains intact after rendering, and can be edited to get ready for the next frame. Such edits are called incremental changes.
For example, a camera fly-through is an ideal case for incremental changes because only the camera position (more precisely, the camera instance) must be changed from one frame to the next, while the rest of the scene is unchanged.
Incremental changes operate on "top level" elements of the scene graph: options, cameras, objects, light sources, instances, groups, shaders, etc. Most elements can be edited while retaining previous values. For example, to turn ray marching off, it is sufficient to incrementally change the options element, and only supply the new value of the "trace" attribute, without redefining the other attributes. Shaders and geometric objects are always redefined from scratch because incremental changes tend to change not just values but the complete structure layout, and in the case of objects would require long-term storage of large amounts of source data not needed for rendering.
The database principle also allows storage of several disconnected scenes or parts of scenes. Deleting scene elements is also possible, but care should be taken not to manually delete elements that are still referenced by other elements.
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