Halliday & Matthiessen (1999: 322):
If the field is defined in relatively broad terms, it may activate most of the general part of the ideation base; but a more restricted field may call on only one particular part of it. In such cases, it may be useful to reduce the whole ideation base to only those parts that are implicated for the particular field; that is, to set up a field-specific ideational semantics. (More generally, this might be a context-specific semantics, including interpersonal and textual as well as ideational meanings). However, the price of this would be to isolate such a field from others to which it is related.