Dialog is the syntactic level of human-computer interaction; it is rather like the script of a play, except the user, and sometimes the computer, has more choices.
Notations used for dialog description can be:
diagrammatic: easy to read at a glance
textual: easier for formal analysis.
The dialog is linked:
to the semantics of the system, what it does
to the presentation of the system, how it looks.
Formal descriptions can be analyzed:
for inconsistent actions
for difficult to reverse actions
for missing items
for potential miskeying errors.