The ANZSCO (Australian and New Zealand Standard Classification of Occupations) taxonomy contains occupation codes/descriptions/concepts that are designed to capture ambiguous occupation responses. These occupations are denoted with the suffix 'nfd' referring to 'not further defined'. There are a total of 336 nfd SCOs in the 2013 ANZSCO taxonomy. While part of the taxonomy, these occupations do not map to the different ANZSCO levels.
Some nfd occupations are rather granular, like Food Trades Assistants nfd. It has the code +8512+00, because it is meant to be the catch-all occupation for the 4-digit SCO Food Trades Assistants.
Other nfd occupations are much more general, like Managers nfd. This SCO has the code +1+00000 because it is meant to be a catch-all for the 1-digit Major Group.
Using Sales Representatives and Agents nfd (610000) as an example, this 6-digit occupation is meant to represent all jobs which cannot be classified more granularly beyond the 2-digit Sub-Major group. Take a look at how this SCO's code is +61+0000, with the first 2 digits representing the 2-digit occupation group Sales Representatives and Agents. There are no digits beyond the 2 digits that allow us to link this SCO to a 3-digit or 4-digit SCO.
Due to the presence of these ANZSCO codes, ANZSCO recall rates will differ when moving between the different levels of the taxonomy (i.e., from 6-digit to 3-digit)