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Scan Depth is the number of sub-directories to drill down into (as it says below that field “how far to drill down”), it has nothing to do with how many domains you have. It also says “-1 is infinite depth”, which is the default and recommended setting. Setting this to any positive number would restrict the scan from digging down into any sub-directory below that number from the level that the scan starts in. You should only change this setting from the default -1 if there is some specific reason that you don’t want the scan to hunt too deeply into all the sub-directories in your scan path.