A list of available AOVs available for the Shadow Matte shader. Each option creates a separate AOV render pass for that component. Note that the AOV must also be enabled in the Render Settings window.
There may be areas visible in the specular reflections which are outside of the background plate; so called offscreen areas. This parameter defines the color used for these offscreen areas; you can link a texture for instance.
Offscreen Color found under Arnold attributes of image plane
The Shadow Matte shader requires more than 1 sample to avoid noisy shadows. This is because it is computing a ratio of shadowed/unshadowed lighting, and to get a good estimate of that it requires multiple samples.