Perhaps just to expand a bit on multi-tenancy... One example is my current employer where we have an instance of Helpmaster serving the IT department. The IT department requires about four security roles (HM admin, IT Manager, Staff, Helpdesk), two sites and five skill groups (FLS, Networks, Applications, Database, Desktop Support). We have different types of jobs (all the ITIL classics: incidents, service requests, change requests) and categorisations (Hardware, Software, Database, Network..). I think of that as single tenancy.
Now, let's consider servicing the requirements of our Facilities & Services business unit with Helpmaster. Their jobs are things like changing light bulbs, fixing air con, fixing a broken door handle, removing graffiti, etc. They will need skill groups for electricians, plumbers, painters, builders, etc. They'll need a different issue hierarchy (unless the root level element is separated by tenancy). They'll need different security roles (Reception, Staff, Contractor, Manager). This is again a single tenant. While it would be possible to run both business units with the one instance of Helpmaster, it would probably be better to create separate installations or look at enterprise scale software. The reason for looking at a true multi-tenancy application would be for senior management to have consolidated reporting or for the transference of jobs between business units (eg: a data point installation might require work to be done by both business units).
But then again, compare to an ISP. An ISP has multiple departments doing vastly different things (sales, support, accounts). I have no doubt Helpmaster could manage that kind of multi-tenancy environment quite effectively. So it depends.