There are 3 system types which are relevant to an ActiveDiscover solution:
Source – system that contains the proposed changes for analysis
Central – connections to Source and Target systems for comparisons. You can use any Netweaver ABAP system from which RFC connections to the Source and Target are configurable. A dedicated Central system such as Solution Manager or the ActiveControl Domain Controller, would suit this purpose.
Target – The system or systems in which proposed changes are to be installed. Would often be a Production environment, but could be any separate system which you want to compare against.