Off site backups typically refers to a backup at another location, this can be a different data center from where your main servers are, or a different provider. The good thing about 99Stack Cloud is it's use of independent providers, adding an additional layer of security when distributing across data centers.
However, there is a drawback too, and that is money. The way you are billed here makes it easy to lose data if you don't pay on time and only have one backup in the same account. I know it's for privacy but still annoying. For comparison, when I got banned from another provider that shall not be named, they let me keep my servers for 4 months after my ban, probably sold my data to scammers in the process, so I do prefer the privacy approach they use here.
Last but not least, for cloud services in general, an offsite backup would also be a backup at your home or office. This is the most important backup and you should always keep your own data close to you and easily accessible. Never trust someone else to manage all of your backups.