Standby - 'A' ports are active, and 'B' ports are in standby mode waiting for a takeover to occur.
Partner - 'A' ports operate as they did in Data Ontap 6.4 and 6.3. However, the 'B' ports become a "proxy path" and operates on behalf of its partner node. When a takeover occurs, the 'B' ports continue to operate for the partner node, the node names do not change, and I/O continues to be processed.
Mixed - mode does 2 things. It combines 'standby' and 'partner' modes and allows all FCP target ports to operate in all 3 states (local, standby, and partner). It accomplishes this by using the 'multi-id' functionality to have multiple FCP portname/nodenames on a single FCP port. This requires the port to operate in Loop mode.
Single_image - was added in Data ONTAP 7.1 and is the default in Data ONTAP 7.2. It allows all ports to host LUNs from both filers, and it makes the filer act as a single FCP device server with only 1 FCP nodename. This mode works on all filers. 7.3 will only operate in Single_Image mode unless there is a previous mode configured before the system undergoes a DataONTAP update. It will take on its previous role; if you want to change the role, it can only be changed to Single_Image and cannot be returned to its mode it was upgraded from or any other previous to 7.3 mode.
Also, at that this point all modes other than single system image are essentially considered deprecated (good to know about for NCDA test reasons but almost definitely shouldn't be implemented in real life for fresh installs).
The questions is in reaction to a number of existing environments, customer info requests/meetings where they make refference to these options and even in the current version of Synergy it is a required checkbox for certain configs.
So to be better able to address the various questions, I first need to read up on the different options.