So the definition I use for field is that the field is the broad, over-arching topic of the text. For example, a government brochure on vaccinations may have a field of 'medicine' or 'public health'. VCAA only mentions "field" in the Study Design, and doesn't define it, and I think some teachers define field as much more specific. Also, some teachers use "domain" interchangeably with "field" but I would be careful doing that, as VCAA doesn't use "domain" in this way.
This definition of field is distinct from semantic fields, which is listed in the Study Design as "semantic fields/domains". A semantic field is a set of words with a related meaning. For example, "wingman", "uncontested possession" and "behind" would all be in the semantic field of AFL.
Of course, a text's field and the semantic fields in a text are related in a way. For example, the government brochure with the field of "public health" may have words in the text from semantic fields of disease, law, education, government etc.