The picture sits in the word field. Not a field of grass — field is the place where the work is really happening, as opposed to the desk, the office, the headquarters. An organization has two kinds of people: the ones at the centre with the paperwork, and the ones sent out to where things are actually going on. A field officer is one of the second kind. Change the organization and you change what the outside looks like — a farm, or a unit of soldiers on the move — but the position is the same: out there, not back here.
From the picture
跟部隊一起在外面的那一層軍官
同樣是「在外面，不在後方」，只是這裡的組織是軍隊，外面就是部隊所在的地方，而 officer 這時候真的是「軍官」。
noun · C1
An officer in the middle band of army ranks — above the junior officers and below the generals — senior enough to command a large unit such as a battalion or a regiment.
校級軍官（校官）
noun · C1
Someone employed by an organization to do its practical work outdoors, at the places where the work actually happens rather than in an office — often work to do with land, farms, or the countryside.
外勤人員；實地專員
以下是外部詞典記錄的用法，原文照列，我們還沒有寫成教學內容。上面的說明才是我們自己寫的；有些用法可能上面已經講過了。
field officernoun
A (para)military commissioned officer who holds a field grade, ranking under general officers but above lower grades and fit to command medium-size units on the field, such as a battalion or regiment; or a non-army equivalent in rank.
資料來源：英文維基詞典 English Wiktionary（CC BY-SA 4.0）。 原文照列，未經改寫。