What do the codes 0, -66, -77, and -99 mean in my data set?

The meaning of the codes is as follows:

0: project variables of type v_000, except for variables that refer to text boxes: the respondent has seen the question, but has not edited it. Respondent management variables of the type “text box”: pre-filled entry where the variable has not been filled.

-66: project variables of type v_000 that refer to text boxes: the respondent has not seen the question concerned, because it was hidden.

-77: project variables of type v_000, except for variables that refer to text boxes: the respondent has not seen the question concerned, perhaps because they aborted the survey or because they were not shown the question/page due to the filter settings.

-99: project variables of type v_000 that refer to text boxes: the respondent has seen the question, but has not edited it.

In relation to text boxes, -99 indicates that a respondent has seen a question, but has not answered it.