The “Abbr.:” field on the “More Options” section of the Source Dialog can be used to provide an abbreviated, or “Short” title for a source.
The abbreviation field can also be used to give an alternative title. For example, if you created a source that is based on the extracts from a parish register of all the children baptised to a couple, you might give the main title as “Extracts of Baptisms for the Children of John and Jane Smith, Middleton, Register of Baptisms 1813-1842”, you could then use “Middleton Register of Baptisms 1813-1842” as the abbreviated title.