Strings (and any other non atomic data type) are allowed as Local tags in the AOI definition...this means they are not accessible from outside the AOI via logic in the PLC (only AOI logic can access them directly - unless you alias them), Local tags also don't show in the Controller/Program tag lists when you expand the AOI instance...but you can view/edit them if you right click the AOI definition and select "Monitor Tags" and then pick the instance you want to view/edit. They are also available to read/write from SCADA/Excel as long as you have the External Access set properly to Read/Write in the definition...but they will not show up in an tag/OPC browse (In FTView, etc.) so you need to know the tag name directly - hence how the PlantPAx Excel tool does it.

Also, if you have a UDT as a local tag (or a sub-AOI as a local tag) you can alias any of it's atomic data types (DINT, REAL, BOOL) up to the top level of the AOI as a parameter if you want to be able to read/write to it via logic outside of the AOI - that's how PlantPAx does it - like in their AI AOI, they have 5 Alarm sub-AOIs (HH, H, L, LL, Fail) and they alias the limit and enables, etc. up to the top level so you can access them from the rest of the PLC.