1. A digital data communication system comprising a transmitter and a receiver, A) the transmitter including: a1) file storing means for storing a plurality of files, and a2) transmission means for repeatedly transmitting the files packetized thereby in accordance with a transmission protocol, the packetized files uniquely assigned identifiers; B) the receiver including: b1) selective receiving means for selectively receiving the packetized files having predetermined identifiers in accordance with a selecting condition set therein and outputting data contained in the packetized files, b2) display data generating means for generating display data according to the outputted data, and b3) selecting condition varying means for varying the selecting condition; and C) wherein c1) the file storing means stores files formed of self-descriptive data which include a reference command for referring another file, and wherein the identifier of each file includes characters directly be selected by the selecting condition varying means, c2) the display data generating means generates display data according to the self-descriptive data outputted by the selective receiving means, and c3) the selecting condition varying means provides an identifier assigned to a file so as to obtain the file specified by the reference command in the self-descriptive data to the selective receiving means.