SureBook

Licence

SureBook is free software, released under the GNU General Public Licence, version 2 or later.

That is the whole licence. There is no separate end-user agreement, and there is no clause anywhere restricting what you may do with the code — imposing one would contradict the licence the plugin is published under.

What that means for you

Use itOn any number of sites, yours or your clients', commercially or otherwise. No activation, no seat count, no licence key.
Read itThe source ships with the plugin. Nothing is obfuscated or minified without its source.
Change itModify it however you like, for yourself or for a client.
Share itRedistribute it, modified or not, provided you pass on the same freedoms under the same licence.

The full text is at gnu.org/licenses/gpl-2.0.html.

No warranty

As the licence states, the software comes with no warranty. It is offered in the hope it is useful, and it is tested seriously — but you run it on your site, taking your customers' money, and that risk is yours.

The paid tier does not change this

When the paid tier exists, it will be a subscription to a service — work done on a server we run — and not a licence to the code. Any code that ships to your site remains under the GPL. Paying gets you access to the service, support and updates through the usual channels; it does not and cannot buy you a more restricted version of software published under this licence.