Each time a developer releases a new app or an app update, they can generate promotional codes that allow users to download an app ahead of release. Generally these codes are provided to press for reviews, and when the generated codes are used up, no additional codes can be obtained until the app is updated.
With an extra 50 codes to go around, developers will have more freedom and flexibility when promoting their apps in the future. While Apple has yet to confirm the promo code increase, developers are already seeing the additional codes in iTunes Connect.
Article Link: Apple Now Allowing iOS Developers to Generate 100 Promo Codes Per App Release