The Difference Between The Social Environment Of A Text And Its Context Of Situation

When we construe the social environment of a text, we are construing phenomena of first-order experience.

When we construe the 'context of situation' of a text — an instance of the culture as semiotic system — we are construing phenomena of second-order experience, that is: metaphenomena.

The cultural context is realised by the language that is projected by interlocutors.  The semiotic context realised by language is second-order experience, whereas the interlocutors doing the projecting constitute first-order experience.

See also here.

Distinguishing Social Context From Cultural Context

The social context of language is of the material order of experience from which sayers and sensers project wording and meaning (the content plane of language).  The social context is a model of first-order phenomena.

The cultural context of language is of the semiotic order of experience.  It is the culture as a semiotic system that is realised as language.  The cultural context is a model of second-order phenomena: metaphenomena.

First Order Field Vs Material Setting

Halliday & Matthiessen (1999: 321-2):

…  the first order field is the social activity being pursued (e.g. instructing somebody in how to prepare a dish, predicting tomorrow's weather, informing somebody about yellow-pages information over the phone) …

Halliday (2007 [1991]: 278):

The setting, on the other hand, is the immediate material environment. This may be a direct manifestation of the context of situation, and so be integrated into it: if the situation is one of, say, medical care, involving a doctor and one or more patients, then the setting of hospital or clinic is a relevant part of the picture. But even there the setting does not constitute the context of situation …

Projections Of Meaning & Wording: Ideas & Locutions

Halliday & Matthiessen (1999: 108):

We have suggested that the relation of projection sets up one figure on a different plane of reality — we refer to this as the second-order or semiotic level. This second-order level of reality is the content plane of a semiotic system. That is to say, the projected figure is projected in the form of ‘content’. We have seen that the content plane is stratified into two levels — semantics (the level of meanings) and lexicogrammar (the level of wordings). Consequently, we would expect projections to be located at either or both of these levels, and this is indeed what happens: a projected figure is either a meaning or a wording. […] We will refer to these [i.e. meaning and wording] in the context of projection as ideas and locutions.

Construing Orders Of Experience

Halliday & Matthiessen (1999: 106):

Throughout the semantic construal of human experience, there is a differentiation between two orders of reality: between the everyday reality of our material existence on the one hand and on the other hand the second-order reality that is brought into existence only by the system of language. This is a contrast between semiotic phenomena, those of meanings and wordings, and the first-order phenomena that constitute our material environment. (Note that the linguistic processes themselves, as apprehended by our senses, are part of the first-order reality; second-order reality is formed of the meanings and wordings that these processes bring into being.)

Hypothetical Material Plane Or Semiotic Plane?

Halliday & Matthiessen (1999: 119):

The logico-semantic relation of condition, which is prototypically construed as a form of enhancement, could also be construed as a kind of projection … Words such as supposing and assuming are verbs of projection which have come to function as conjunctions in conditional figures; while other words such as imagine and say retain more of their projecting force. … This is an uncertain region in which a figure hangs in the air, so to speak, suspended between the hypothetical material plane and the semiotic one.