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Clicking the Edit button brings up a panel on which you can either delete current FoR labels (if they don’t apply to your principal interests; use the X icon) or search for key fragments of text to find labels that better reflect your interests (select label and click Add).
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The pre-population of Fields of Research use the 4-digit codes. One strategy is to choose 6-digit sub-codes within a category to describe your research more specifically. If you type the 4-digit code of one of your labels into the Search box, you can choose one of the 6-digit FoR codes in that category for more specificity.
You can find the complete list of 6-digit Fields of Research codes at the Australian Bureau of Statistics.