UE class determines the speed at which data can be transferred in uplink and downlink directions. For GPRS & UMTS, most of us already are aware of it. LTE have class recomendations as well. Lets have a look at 3GPP proposed UE classes for LTE. 

Each radio access technology has defined specific “classes” of terminals in terms of radio capabilities. E.g. in GPRS the “multislot classes” are defined, in UMTS R’99 different dedicated bearer classes are defined and for HSDPA and HSUPA 12 & 6 physical layer categories respectively are defined. 

 The definition of UMTS R’99 UE classes lead to 7 DL classes and 7 UL classes for FDD out of which only 2 DL and 3 UL classes were commercially realized. 

For HSDPA , out of the 12 defined categories only approx. 4 will be realized in commercial HSDPA platform products. A similar situation is likely for HSUPA as well as for the combinations of HSDPA/HSUPA.

Generally the aim to mandate certain essential functions/requirements can help to simplify the system definition as well as the realization options (e.g. mandating 20 MHz of DL reception as well as 20 MHz UL transmission bandwidth significantly reduced the E-UTRAN system complexity). 

To avoid unnecessary UE mandatory features but instead defining a limited set of UE radio classes allows simplification for the interoperability testing.

Based on above, 3GPP decided to limit the combination of radio capabilities to a clearly defined subset and ensuring that a given set of parameters is supported by certain UE classes as well as networks for rapid E-UTRAN deployment. It seems unrealistic to mandate only one single UE class which always mandates the maximum capability.

In order to address the different market requirements (low end, medium and high end), the definition of the following UE classes are proposed for LTE:

The aim is to ensure on the one hand that high end E-UTRAN UEs, supporting data rates representing state of the art level and competitive with other radio technologies are defined, while the medium and lower data rates aim to reduce implementation cost for chipset/terminal vendors and allow adoption of most cost efficient solutions for different market segments. It is expected that the support of the high end data rate terminals is ensured from the very beginning. 

source: 3GPP standard