* Ausfüllen means to fill out or complete something. It is used for forms, questionnaires, blanks, etc., where you are providing information by writing or typing in the empty spaces.
* Auffüllen means to fill up or top up something. It refers to adding something to a container or space that is already partially filled, bringing it to its full capacity.
Here are some examples to illustrate the difference:
Ausfüllen:
* Bitte füllen Sie den Fragebogen aus. (Please fill out the questionnaire.)
* Ich habe das Formular ausgefüllt und abgeschickt. (I filled out the form and sent it.)
Auffüllen:
* Bitte füllen Sie den Tank mit Benzin auf. (Please fill up the tank with gasoline.)
* Der Pool muss wieder aufgefüllt werden. (The pool needs to be refilled.)
In a nutshell:
* Ausfüllen is about providing information and completing something.
* Auffüllen is about adding something to make it full.