That you can change in preferences, right? And it usually excludes, by default - changeable, system files and the sort.
(Used Backblaze for one month happily during trial period, decided not to switch because of their, imho ridiculous, file/version retention/deletion policy. And an unhappy Crashplan customer for ~5 years actively looking around for similar feature set but sanely usable client and and to switch to. Arq doesn't fit the bill. I wish Backblaze wouldn't delete the files/versions/disconnected drives that soon if not not ever. And also website only restore actually is funny)
Most of the excluded defaults cannot be changed / are read-only in the preferences. System files are some of it, but it includes others as well, such as all Applications. By default it also includes all ISOs and DMGs (this one can be changed but IMO is an insane default) — I store quite a bit of important info in encrypted DMGs.
(Used Backblaze for one month happily during trial period, decided not to switch because of their, imho ridiculous, file/version retention/deletion policy. And an unhappy Crashplan customer for ~5 years actively looking around for similar feature set but sanely usable client and and to switch to. Arq doesn't fit the bill. I wish Backblaze wouldn't delete the files/versions/disconnected drives that soon if not not ever. And also website only restore actually is funny)