In Gnome shell overview search this extension allows you to search 
only in specific category (Aplications, Settings, Places & devices, 
Recent Items, Contacts). It tries to match first word of the search 
term against prefixes of category names.
Usualy you need only first letter of a category, but if in some 
localization more categories have the same first letter, you can 
distinguish between them with any category name prefix.
This functionality is used only when you search term cosists of 
at least two words and the first word is prefix of a category name.
Sample searches (try some of these to get the idea how it works):
setting key
set keyb
s k
recent avi
re pdf
r jpg