A simple example of a valid Scheme search is:
(string=? (item-priority item) "A")
This uses the function item-priority to get the priority from item, where item is available during searching and represents an item being matched.
If you take a look at https://beorgapp.com/manual/library-org/ you'll see defined the other functions which can be used in matching (under the Org headline Items). You can then use Scheme logic functions such as or to combine these.
Niels Kobschätzki