Different STM32F405RGxx MCUs

I am trying to find the technical difference between the following STM32 chips: STM32F405RGT7, STM32F405RGT6, STM32F405RGT6W and STM32F405RGT6TR.  I know the details of the STM32F405RG family, but I cannot find out what the T6, T7, T6W etc end designation means from a technical perspective.  Unfortunately this information is not in the specifications, that I can see.  So, what does the T6, T7, T6W mean?

Thanks :)

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