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.