The search terms can be combined with the keywords AND, OR and NOT. For example:

Searching for: Augustinus OR Confessiones : finds all the texts in which either Augustinus or Confessiones appears
Searching for: Augustinus AND Confessiones : finds all the texts in which Augustinus and Confessiones appear at the same time
Searching for: Augustinus NOT Confessiones : finds all the texts in which Augustinus but not Confessiones appears

Searching for text in quotation marks finds the text in the order entered for the words searched for. For example:

Searching for: Lettres de Saint Augustin : finds all the texts in which Lettres or Saint or Augustin appears
Searching for: "Lettres de Saint Augustin" : only finds texts in which Lettres de Saint Augustin appears exactly like this

1.