Halliday & Matthiessen (1999: 99, 102):
The view from the grammar brings into relief a number of relevant factors relating to participants and circumstances. There are certain special subcategories of these, in the grammar, which are distinguished by the fact that they embody features of interpersonal or textual meaning:(1) interpersonal: questioning
interrogative: who, what, when, where, how far, how long, how, why
(2) textual: cohesive(i) referring, personal: he, she, it, they; demonstrative: this, that, now, then, here, there, thus
(ii) generalising: lexical items such as person, creature, thing, stuff, affairThese are significant in their own right because they are critical to the construction of discourse: the textual ones provide internal cohesion, while the interpersonal ones construe dialogic speech rôles. They have further significance in that they reveal by reactance the major subclasses within the general classes of participant and circumstance.