The TURF Top/Down Reach Calculation tool can be found at:
Start Reach: This is the reach relative to all the people who has participated in the survey - i.e. started the survey. This includes users who have started but not completed the survey.
Completed Reach: This is the Reach relative to all the users who have COMPLETED the survey i.e. gone through the last question in the survey.
Respondent Reach: This is the Reach relative to all the users who have CHOSEN AT LEAST ONE RESPONSE to the given question.
The "Completed Reach" and the "Started Reach" takes into account "Non-Respondents" -- users who did not choose any option, whereas the "Respondent Reach" takes into account all the users who've at least chosen one of the available options.
In cases where a large number of people go through the given question, BUT DO NOT COMPLETE THE SURVEY, the Completed Reach is not reliable. This is because the total number of people who choose a particular option can be greater than the total users who actually complete the survey. In such cases you should use the "Started Reach" for measurement.
This feature is available with the following license :