Halliday & Matthiessen (1999: 321, 322):
In most contexts, there is both a first order field and a second order field — 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) and the second order field is the 'subject matter' the activity is concerned with (e.g., the ingredients and methods of cooking, meteorology, construction businesses, international travel). So for instance, in a context where a telephone operator provides a caller with information there is (i) the social activity of exchanging information as a service and (ii) the area of information, e.g. copying & printing services.…a full account of field would include a typology of the possible first and second order values that occur in a culture. Such a typology would show how closely various fields are related — how they form families.