If respondents to a postal questionnaire are also offered the option of answering by web survey, this is called

Offering postal respondents the option to answer instead by web survey is a mixed mode survey.

It combines two or more data-collection modes within the same study.

Optional survey is not a standard methodological label.

Multiple survey does not name this single mixed-mode approach.

Selective survey is not the recognised term here.

Mixing modes can raise response rates but risks mode effects.