The conditions attached to each the immigration categories, which appear as headings on the passenger lists (Full Paying, Free, Assisted, Remittance) are listed in the article below. From what I can make out,
Assisted passengers were required to repay a set amount over time, whereas female domestic servants listed as
Free did not.
1870 'IMMIGRATION.', Maryborough Chronicle, Wide Bay and Burnett Advertiser (Qld. : 1860 - 1947), 8 March, p. 4. , viewed 06 Jul 2017,
http://nla.gov.au/nla.news-article148019696M

Add: It also looks as though
Assisted passengers were required to pay half the amount upfront, but
Free did not.