For multi-part packages, like many logic gates, the schematic symbols need to be broken apart, since you rarely use them all in one place in your schematic. The same applies to dual or quad op amps.

The symbols are suitable for use in dynamic displays, as on cathode-ray tubes (CRT's), and static displays, such as graphic panels. Though the standard may make use of standard symbols now used for ...

The technical report, ISA-TR5.1.01-ISA-TR77.40.01-2012, Functional Diagram Usage, expands upon and illustrates usage of function block symbols and functions, and prepares examples in development of ...