The file descriptor is to support capabilities?

Source: https://nlnet.nl/project/Capability-based-RedoxOS/

"[..] In this project we will replace Redox's internal file descriptor representation with capability descriptors, optimized for both security and performance. This will provide a foundation for capability-based security on Redox, and possibly capability extensions from other UNIX-like systems, while also supporting POSIX-style file descriptors for application compatibility".