My 3rd great grandparents John Billin and Mary (nee Symons) with their four children (including my 2nd great grandmother Emma Symons Treloar Billin, aged 5) left Plymouth, Devon, England on 11 May 1849 and arrived at Port Adelaide, South Australia on 23 Aug 1849.
On one of the documents associated with that voyage (see below) there is a column headed "From what County selected" and the entry for John Billin lists "Middlesex".

John Billin was certainly a Tailor after taking over his deceased father's business in Helston, Cornwall, England on 19 Jun 1839 but that business folded on 16 Apr 1845 (see below).

The names and ages of the family all match up with Helston christening records so I am very confident that this is the family in which I am interested.
I am keen to know whether the Middlesex reference indicates that John and his family had moved to Middlesex for perhaps a few years before emigrating (for which I have no other evidence), or whether an alternative theory that John may just have gone there "for an interview" might be plausible.
I suspect that gaining a better understanding of the "selection process" for South Australian immigrants at that time and place might be helpful.
How should the entries in the "From what County selected" column on a document such as this be interpreted?