dtr-2tog stands for a stitch that is made by working 2 double treble legs together.
dtr-2tog stitches can be symmetrical (both legs have the same length and are worked in the stitches that are as far behind and below as they are ahead and below) or asymmetrical (one leg is further behind/ahead or below as the other)
In the case of the dtr_-3-3 in FLO/dtr_+3-3 in FLO/<2tog it is specified that the first leg is a double treble in the front loop of the stitch 3 behind and 3 rounds below the next st and the second leg is a double treble in the front loop of the stitch 3 ahead and 3 rounds below.