Subscribe.jsscript is a hosted script that will be used, among other things, to display multiple authentication elements on your site (signin/signup buttons, ...).
Subscribe.jsSDK will handle the rest!
Subscribe.jstag itself. Like any script out there, the
Subscribe.jsscript can be loaded in a lot of different ways.
cdnsubdomains if you are not comfortable with wildcards) and
assetssubdomain) to your whitelist in order for the elements to operate normally.