The time of the triggering event is the instant at which Gaia detected a significant change from a constant magnitude, and that depends more on Gaia's scanning law than on astrophysical events in the source. Notably, for eruptive events, the time of peak brightness may be either after or before the triggering time of the alert.
Time of observation is in barycentric coordinate time (TCB) rather than in UTC. Time of publication is in UTC.
The sky position may either refer to a source in Gaia's own catalogue, or to a source in an external catalogue (e.g. SDSS) used as a reference for combining Gaia observations. Where the position comes from Gaia's catalogue, it is derived from a single, Gaia observation at the triggering point of the alert; this is not an astrometric measurement to the full precision of the Gaia main mission.
Magnitudes are in Gaia's "G" band. These are unfiltered, white-light observations in which the pass-band is defined by the instrument response. The magnitudes in the table are derived from a preliminary calibration of the photometry. The forthcoming photometric-catalogue from the Gaia mission will provide more accurate magnitudes based on a proper calibration.
For each Alert, on its webpage, we present the lightcurves in a figure, and also via a downloadable CSV file. Our published lightcurves use a timestamp extracted from the reference time encoded in the transitId, i.e. the time of observation in the first strip of detectors in the astrometric field (AF1, see e.g. Fig 4 in Gaia collaboration; Prusti et al. 2016). The TransitId is a numeric field which encodes a number of values: the reference pixel coordinates in AF1, the telescope, and the CCD row in which the object was observed. It uniquely identifies the transit of a source on the focal plane. We convert this AF1 timestamp to Barycentric Coordinate Time (TCB: hence the conversion includes the light travel time from the spacecraft to the Solar system barycentre).
The column headings are: Date(TCB), JD(TCB), average mag (i.e. mag averaged across all CCD strips). Note that it takes approximately 44 seconds for a source to cross 9 AF CCDs
In the lightcurve data, we have epochs where no numeric value is given for the magnitude. In this instance we report two distinct cases with a text label in the CSV file, and a different symbol in the figure.