Questionnaire variable field is a new mandatory field for questionnaires in Designer.

Previously, the Questionnaire name field dictated the name of both the questionnaire and its main export file. Now, the questionnaires have two fields in Designer: the Questionnaire name field is a human-readable name that can contain any characters you wish, in any language, and can be rather large.
The Questionnaire variable field is an identifier that will be utilized by Survey Solutions during export of the data for naming the main data file and should follow the same rules as applied to variable names and roster names. This makes the naming of files more rigid and independent of the language in which the questionnaire was designed. For questionnaires to be valid, both identification fields must be populated. That means that this field must be filled for new old questionnaires as well as newly created ones.