A specification that allows phones (and other mobile devices) to output stronger radio signals in specific radio frequency bands and situations. This improves coverage and indoor signal performance.
Specifically, HPUE is a name for 3GPP Power Class 2 as applied to LTE technology operating in band 41 (LTE 41). Prior to the introduction of HPUE, LTE 41 was limited to Power Class 3. HPUE / Power Class 2 allows phones to transmit stronger signals when signal conditions are otherwise poor.
See: Band 41
Phones with HPUE are still subject to the same SAR limitations as other phones, that keep radio radiation levels within what the FCC has determined are safe levels for humans.
HPUE allows LTE networks in band 41 to achieve coverage similar to LTE networks in lower-frequency bands, given a network with the same density of towers. Band 41 is unusually high-frequency and therefore has relatively poor indoor coverage and outdoor range without HPUE.
Sprint and China Mobile are the two largest networks in the world that have deployed LTE 41, and the two largest proponents and users of HPUE.