Field is the word the site uses when the point is not what an ecology contains but how it acts. An ecology names what the whole is made of: participants, conditions, resources, relationships. A field names the same whole as something that acts. It shapes what a person can see, reach, and become, and it changes as they act in it.
The field forms the participant; the participant alters the field; the altered field acts back on the participant and on others.
A field can be a whole ecology or one domain within it, such as a person’s work, town, or family: the fields where they have standing. Field ecology uses the word for the shared whole that everyone draws on, as distinct from the part any one person can reach. In ordinary English “field” also means a line of work; the site says “occupation” for that. See What Is an Ecology?.