For most FDTD sales, we generally package both 32 & 64-bit together. All new releases will be 64-bit only and compatible with only Windows 7 & 8.
I believe the main differences between the 32-bit purchased & free are: OptiMode utility & Customer Support. Aurelien Duval would be the best person to provide you with a detailed comparison list (email@example.com).
Differences between 32-bit & 64-bit are listed here: https://optiwave.com/products/component-design/optifdtd/optifdtd-applications/64-bit-fdtd/
If you have more specific questions about your license, please email us directly: firstname.lastname@example.org