What is the difference between Restor Factory Default and Format Memory Card

I had an issue today with resuing an S7-1515-2PN, it had previously been used for a project in TIA 15.1, the new project was in TIA14. I got a repeated error that the project was not compatible, the onltine project was TIA15.1 and the offline was 14.0.0.0, to get around this I ultimately installed a dymmy copy of a TIA14 project and formatted the memory card. 

My question is why a restore factory defaults did not wipe the PLC out? After my initial install of the TIA14 project, the TIA15.1 project name was still on the PLC and I had the error above. Format memory card was greyed out unitl I loaded the dummy TIA14 project. Why does reset factory defaults not zero out the PLC as the name would suggest, and why is format memory card the feature that worked?

Hello U_cf7082a6-618f-4...,

This is because in the S7-1500 the program is stored on the Memory Card. Therefore, even if you do a factory reset of the PLC, the program will continue on the Memory Card until you delete it.