From the AT series, it's a Attenuator.A solid enclosure of Tape & Reel (TR) surrounds optical attenuator.Shipping overseas is convenient with the RF Attenuator packed in 0603 (1608 Metric).
AT0603C10ECATD Features
AT series
AT0603C10ECATD Applications
There are a lot of American Technical Ceramics AT0603C10ECATD Attenuators applications.