Formerly BitTorrent Sync, Resilio was spun out into its own company in 2016.
Resilio Sync synchronizes files using BitTorrent, The user's data is stored on the user's local device instead of in a "cloud", therefore requiring at least two user devices, or "nodes," to be online to synchronize files between them. Resilio Sync encrypts data with an Advanced Encryption Standard AES-128 key in counter mode which may either be randomly generated or set by the user. This key is derived from a "secret" which can be shared to other users to share data. Data is sent between devices directly unless the target device is unreachable (e.g. behind a firewall), in which case the data will first be relayed via an intermediary node. Many devices can be connected simultaneously and files shared between them in a mesh networking topology.
There is no limit on the amount of data that can be synced, other than the available free space on each device.
This lead me to Comparison of file synchronization software (wikipedia)