What are the definitions of the next? and unless-fit? values of the float property? http://www.princexml.com/doc/8.1/properties/
They are modifiers that apply to other float values. For example "float: top next" will float to the top of the next page, or "float: column-top next" will float to the top of the next column.
The unless-fit modifier will leave the content where it is if it fits in its natural position, otherwise it will apply the float to avoid leaving a gap at the bottom of the page or column. At the moment you can't combine "next" and "unless-fit".
These values are relatively new, in the past Prince had keywords like "top-next" and "prince-column-top-if-necessary". The CSS working group is still standardizing the specification for the float property, so there may be further changes in future releases.