Hi Shikhar,
I work along with Arnaud. Thank you for getting back to us.
With regard to your concern, there can be ad slate at the start of a AD BREAK or in the middle. If the ads length doesn't fill the ad break space allocated to it, a slate will fill the remaining space of the ad break. If for some reason, an ad is not found by the server or throw an error, a slate will play in place of the ad.
Regards,
Teejay Pimentel