SBackup incremental from previous increment or previous full backup?

It seems to make sense for SBackup to do an incremental backup from the previous full backup, but the documentation that I have found is not explicit.

So, in the case of a series of incremental backups, is each increment based on the previous increment, or based on the most recent full backup?

That is, if I want to keep a full backup and the most recent increment, am I able to delete all the intermediate increment folders?

THanks!