What is meant by the first and last shot in the record?
There are 40 laser shots in one second which are associated with the transmit pulses. The first shot is from the laser, the last shot is the 40th record of that transmitted pulse.
The GLAS Altimetry User Guide explains it like this:
"Thirty-nine delta times are provided, which the user adds to the full UTC time of the first transmit time in each record, to calculate the 40 laser shot times."