It depends on the ToS. DDoSing might be considered unreasonable use.
But if you're using VPS to stream 4K content 24/7, that would be heavy and reasonable use.
Similarly, if I take the unlimited Dropbox plan and resell it, that's probably against the ToS.
If I'm uploading 50TB of blu ray rips for backups, that's... Heavy use but entirely acceptable based on what they're advertising.