In sequential hypothesis testing, the sample size is not fixed in advance. Instead data are evaluated as they are collected, and further sampling is stopped in accordance with a pre-defined stopping rule as soon as significant results are observed. Thus a conclusion may sometimes be reached at a much earlier stage than would be possible with more classical hypothesis testing.
Since the sign test is non-parametric, we do not care about the distribution of the samples. We just want them to be continuous.
Read the reference for the method description and assumptions:
Sequential Methods and Their Applications
by Nitis Mukhopadhyay, Basil M. de Silva
Jan Motl (2020). Sequential sign test (https://www.mathworks.com/matlabcentral/fileexchange/71633-sequential-sign-test), MATLAB Central File Exchange. Retrieved .