This Technical Specification specifies a number of options for the identification of individual mail aggregates, defines methods of constructing an aggregate identifier and specifies one required and a number of optional methods by which this identifier may be associated with (affixed to) the mail aggregate or its temporary container. This Technical Specification also identifies a number of mail aggregate attributes, or characteristics of aggregates (as distinct from the attributes of the individual mail items and lower level aggregates within them), which may be useful to communicate between parties handling the mail aggregate concerned. The attributes are not defined in detail in this document. Precise definitions and encoding formats for them will be developed over time, as a result of practical experience of use of this Technical Specification. The definitions will be included in an appropriate reference specification, such as UPU S25 and M82, which serve as baseline definition documents for attributes used in postal industry communications about postal items, batches and postal receptacles.