A 'Partially Known Environment' is one where the penetration tester has some information about the target system, such as maybe the network diagrams or configurations, but does not have complete knowledge of the systems and infrastructure. This type of environment is realistic as it mimics situations where a security professional may have insider knowledge but still needs to discover additional vulnerabilities. An 'Unknown Environment' implies no prior knowledge of the system. A 'Known Environment' is when the tester has full knowledge of the system. 'Reconnaissance' is not a type of environment, but rather a phase within penetration testing where information is gathered.