The MUP generated information is divided into a number of subfields which may be either of fixed or variable length. For a given message type identified by a unique message heading, the presence of a given subfield may be either mandatory or optional.

Subfields which have been declared mandatory for a given message type appear in all messages of that type. If no explicit declaration is made, the field type is to be interpreted as mandatory.

Subfields which have been declared optional for a given message type only appear when required in messages of that type. The presence or absence of each optional fields is indicated by the state of a field indicator located in an indicator field, which in this case is a mandatory subfield.

Subfields which have been declared fixed length for a given message type contain the same number of bits in all messages of that type.

For subfields which have been declared variable length for a given message type the number of bits may vary between messages of that type. The size of a variable length subfield is indicated in an immediately preceding fixed length subfield.

For a given type of message the various types of subfields are transmitted in the following order:

Within each of these two classes, the order of subfield transmission is, in general, as follows:

Within each defined subfield the information is transmitted with the least significant bit first.

Spare bits are coded «0».