display units is a special property that specifies the a comma-separated list of units or formats that a property should use in display.
When used on property pages that have a datatype that supports units, the effect is that all printouts will show the values only in the given units, and only in their given order. If a property does not specify any display units, then it will appear converted into all units supported by its type. You can add this special property to a property page to limit the display, or to control the unit name used (e.g. display "mins" instead of "minutes"). The additional unit conversions appear in the tooltip and factbox and some search results.
For example, see on ontoworld.org how ow:Property:Age displays ages in years and converted into seconds, but not into other time units like days, minutes, or hours that its ow:Type:Time also supports.
Note that even if you limit the display of units, you can still specify a property value using any unit that the underlying type supports.