Good question - here is a good answer:

The U refers to the Ultra-High Speed (UHS) designation which is available on some SDHC and SDXC cards. The following ultra-high speeds are specified:

UHS-I cards, specified in SD Version 3.01, support a clock frequency of 100 MHz (a quadrupling of the original "Default Speed"), which in four-bit transfer mode could transfer 50 MB/s.

UHS-I cards declared as UHS104 also support a clock frequency of 208 MHz, which could transfer 104 MB/s. UHS-I is the only class for which products are currently available.

Double data rate operation at 50 MHz (DDR50) is also specified in Version 3.01, and is mandatory for microSDHC and microSDXC cards labeled as UHS-I.

UHS-II cards, to be defined in Version 4.0, further raise the data transfer rate to a theoretical maximum of 312 MB/s.

UHS memory cards work best with UHS host devices. The combination lets the user record HD resolution videos to tapeless camcorders while performing other functions. It is also suitable for real-time broadcasts and capturing large HD videos.

Cards that comply with UHS show UHS-I (this is the '1' that you were wondering about or UHS-II on the label, and report this capability to the host device. Use of UHS requires that the host device command the card to drop from 3.3-volt to 1.8-volt operation and select the 4-bit transfer mode.

Does that help? Confused?

OK. U is ultra high speed type 1 - mega fast transfer rates needed for video - 50 mb sec. 11 (2) is theoretically 5-6x faster. They haven't made any yet, but are planning for it.

OK, now go to sleep.

Cheers
Take 5