Alternate timeline(covers information from an alternate timeline)
Ensign Thomas was a Starfleet crewmember aboard the USS Enterprise-D in an alternate timeline.
He was paged to report to the combat information center over the ship's intercom shortly after the ship encountered the USS Enterprise-C near a temporal rift in 2366. (TNG: "Yesterday's Enterprise")
It is not clear if Ensign Thomas was a crewmember in the "regular" version of the timeline.