The last render queue that is treated as opaque. This value corresponds to a render queue of 2500.
Render queues in [0, GeometryLast] range are treated as opaque objects (sorted based on Camera.opaqueSortMode to reduce render state changes), whereas queues in [GeometryLast+1, 5000] range are treated as semitransparent objects (sorted based on Camera.transparencySortMode, typically back-to-front).
Additional resources: Material.renderQueue, Shader.renderQueue, subshader tags.