Halliday & Matthiessen (1999: 193):
Discrete semiotic abstractions: intermediate between semiotic objects and (non-discrete) semiotic abstractions. These include non-personalised ‘facts’ and ‘cases’, mental entities like ‘thoughts’ and ‘fears’, and speech functions ‘questions’ and ‘orders’; they are bounded and cannot function as Sayer but can accept a projection as Qualifier (e.g. the order to retreat, her anxiety that she may be disqualified).