It is based on MPEG-4 Part 12.
3GP files can be identified by the presence of one or more 3GP brands in the "ftyp" box (see boxes/atoms format for information about brands). There are a large number of such brands. They apparently all start with "
3g", followed by a letter. An example is "
3gp9". Note that brands that start with "
3g2" are for 3G2 format.