If you are using a subdomain or sub-subdomain (etc.) for your RushFiles solution, you will need the certificate for the domain or subdomain above to get a trusted SSL connection to the web interface.
As an alternative, we can setup a sub-subdomain for your desired subdomain, but this has to be done manually by RushFiles, so make sure that it is agreed on in advance.
Example:
If I wanted to install with test.rushfiles.com, I will need a *.test.rushfiles.com SSL certificate and a certificate that covers test.rushfiles.com like *.rushfiles.com .
Otherwise, I could use a sub-subdomain like cloud.test.rushfiles.com, which would then be covered by my *.test.rushfiles.com certificate, but as written above, it is a manual process and can only be done by RushFiles.