The survey platform supports Save & Resume with or without a participant list. When a person completes a survey that doesn't involve a participant list, when they click Resume Later, they are asked to enter a unique username, a password, and optionally their email address and submit the form. They can then later click on the original survey invitation link, and from the top of the page, they can click "Load unfinished survey" they will be prompted to enter their name and password, and from there, they can continue the survey.If they opt to enter an email address on the "Save and resume later" form, a link will be emailed to them that they can click on to complete the survey at a later time. If a survey involves a participant list and the survey is set to anonymous, if the respondent "Saves and resumes later", and returns to the survey from another device, they will need to enter their token first. Once their token has been verified, they can then click on the Load unfinished survey and login in to load their responses. If they return from the same device, they will be taken to the place in the survey where they left off previously.If a survey involves a participant list and the survey isn't anonymous, and you set "Enable token-based response persistence" to "On" under Survey Properties->General Settings: Tokens panel, if the respondent closes the survey, and then later returns to the survey by clicking on the survey link (which contains their token), then the survey with their previous answers will be reloaded.
We always recommend you try out several options -- you can activate a copy of your survey with various settings related to tokens, create a small participant list of testers, send invitations out to your test group, and see which configuration for save and resume later works best for you.