It will link the time frame between the original manga and the Clear-Card one.

It has been announced that bundled with the newest volume of “Sakura Card Captor: Clear-Card” Manga an OAD will come with it.

This animation will serve as a link betweent he original series and the sequel:

Morio Asaka (Sakura Card Captor) is directing the animation at Studio Madhouse.

the volume, and the OAD, will come in September.

Via: ANMTVLA (Spanish)