Halliday & Matthiessen (1999: 22):
Typically processes of this type leave their traces in the form of marking, as marked/unmarked oppositions. The original member of the set remains the unmarked one. (One could say 'the unmarked term in the system'; but this formulation assumes that the offspring combine with the parent to form a system. Sometimes they do, but not always.) In these first set of examples, the unmarked mapping is that of participant ^ noun; when the noun realises some other element in the figure, it is a marked variant (grammatical metaphor). Presumably many 'unmarked' variants originate in this way, although in most instances we no longer have the evidence which would enable us to judge.