A TimeMachine hard drive can be erased and used as normal for other purposes. It's not a "special" drive, if that's what you are asking.
Few people discuss TM because it mostly "just works". You can run into problems though if you need a bigger TM drive than say, a TC came with. Or worse, if your machine has multiple volumes, and you need a TM space bigger than any HD currently on the market.
iFixit has guides on how to replace the TC internal drive, say to a bigger one. Considering Apple's habit of going for compact size instead of cooling capacity, definitely go for a 5400 or 5900 green drive.
If you need more than 14TB, then you need a RAID array for backups. A NAS is the best option, though there are others. Like USB multibay drive enclosures.