This requires a bit of a longer discussion.
The upshot is this: when you request a list of all sites, you get *at most* 100 back. If the number of sites is equal or exceeds 100, you wouldn't know how many more there are (as the OnPremise feedback won't tell you i think). If on the other hand the returned array count is < 100 (but > 0) you know you got them all - in which case, you can return the cached sites and save the caller another API call.