A Side is a feed within a feed. Most trading venues produce their raw feeds as a pair of UDP multicast feeds. We refer to these two UDP multicast feeds as the A-side and B-side.
Two feeds are used because UDP is a lossy protocol, and this approach delivers redundancy in case packets are dropped in the path. Sometimes one side is faster than the other, which might motivate users to watch the faster feed only, even if it is lossy.
Potentially you might also count other delivery channels of the same feed as being a different Side, e.g. a microwave (RF) feed as opposed to A and B sides delivered over a terrestrial circuit.
In Beeks Analytics feed definitions, a Side sits in the hierarchy as follows:
Venues
Markets
Feeds
Channels
Side
Beeks Analytics for Markets aggregates by Feeds, Channels and Side.