By default, each Target created is configured so that Acunetix will accumulate data about the target such as URLs, vulnerabilities, and other relevant information, and this is stored in a knowledgebase for the Target. For each subsequent scan against the Target, Acunetix will make use of the Target knowledgebase to give a head start to the scanner, by for example providing a list of URLs which have been discovered in previous scans. This will improve the accuracy of the scanner over time as it collects "knowledge" about the web application, especially in the case of Targets which use paths that are not reachable in a predictable or consistent manner.
You can disable the use of the knowledge base data in future scans if, for some reason, you wish to retain a scanner behaviour that only follows URLs which the crawler can reach during a scan. Once the knowledge base feature is re-enabled, future scans will then again make use of any data collected in previous scans.
You can use the "Delete knowledge base" button to delete all knowledge base data for your Target.