I'm just not sure what you mean by "mandatory" here. Do you mean that manufacturers were required to ship cards formatted with exFAT, or they wouldn't be allowed to manufacture SD cards? I actually don't understand what you said, I'm not trying to be misleading.
They mean that exFAT is the only filesystem format a host device that accepts SDXC cards is required to understand, in order for that device to be certified as SDXC-compliant / get to put an SDXC logo on the box. (Also, that an SDXC card that came formatted as anything other than exFAT wouldn't be able to put the SDXC logo on the box.)
By formatting your SDXC card with any other filesystem than exFAT, you won't be able to use it in all the devices that are "minimally compliant" with the SDXC spec in that they implement exFAT support and nothing else. Where "minimally compliant" devices include: literally all digital cameras, all portable game consoles, etc. — pretty much the whole ecosystem of devices that would ever give you a reason to own any SDXC media in the first place.
Thanks for this. I still didn't understand what was meant. To be certified as taking an SDXC card (and by that to have the right to display the logo), a device was required to be able to read exFAT.
Mandatory in this case does not mean that it couldn't be formatted with something else, it doesn't mean that devices weren't allowed to read cards formatted differently, and it didn't mean that manufacturers or distributors of cards were forced to use exFAT, as long as they didn't care about the logo, and weren't concerned about customer support being overwhelmed by people screaming that their new card was broken. It does mean that if Linux couldn't read/write exFAT, it would be incompatible with most available devices, and people would be screaming about how their Linux was broken.
Extra wrinkle: the filesystem you have to use changes based on the capacity. SDHC and SDXC are basically the same protocol, but one mandates FAT32 and the other wants exFAT. This is based on the same 32GB limit that Dave Plummer snuck into Windows NT on a whim[0], even though FAT32's structures can go up to 2TB and Windows can totally parse filesystems that large.
Some devices actually will take reformatted cards that are "technically" "out-of-spec". You aren't supposed to be able to use SDXC cards with FAT32, but devices that never bothered with exFAT will still accept them. This is in fact the only way you even can use a large card on a Nintendo 3DS.
