Format should be pretty straight forward. By decimal notation, I'm pretty sure he means in geographic WGS84. I doubt your data can get too close to the poles either...so allowed latitude is somewhere short of -90 to 90 (-85 to 85 is common) and allowed latitude would be between -180 and 180. Depth is probably in positive meters rather than negative meters.
en.wikipedia.org/wiki/Comma-separated_values
So your format would be a file name *.csv with something like the following text. With a fixed specification, I don't know if Matt expects the data to have a header row (it is optional):
Latitude, Longitude, Depth
52.0, 5.0, 30.0
...