Halliday & Matthiessen (1999: 43-4):
The general form of a realisation statement is 'realisation operator + one or more realisation operands', as in 'insert Senser' and 'conflate Medium and Senser'. The operators used to specify functional structuring are insert, conflate, and preselect. The first operand is always a semantic function (role); additional operands may be functions or features. The realisation statements we will make use of in our representation of ideation base information are as follows:
(1) Presence of functions in the structure: the presence of a function in a function structure is specified by inserting the function into the structure; the operation of insertion is symbolised by '+'; e.g. +Actor, +Senser, etc.(2) Conflation of one function with another: one function from one perspective is conflated with a function from another perspective — they are identified with one another. Conflation is symbolised by '/'; for example, Medium/Senser means that Medium (ergative perspective) and Senser (transitive perspective) apply to the same element of a figure.(3) Restriction on the type of phenomenon that can serve a particular function: this is stated by preselecting one or more features from the unit serving that function; preselection is symbolised by ':', e.g. Senser: conscious being, which means that the participant serving as Senser is restricted to the type 'conscious being'. (Since 'conscious being' is more delicate than 'participant', the fact that a participant serves as Senser can be inferred from the statement 'Senser conscious being'. In general, it is only necessary to state the most delicate or specific restriction along any subsumption path in the system network.)