transistor characteristics - Ube0 number meaning
UBEO is the base to emitter voltage with unconnected or "open" collector terminal.

A bipolar junction transistor has three terminals, base, collector and emitter. Whichever terminal is not connected for the rating is replaced with open when left unconnected. Hence similar identifiers for UCBO, UCEO.

UEBO = -UBEO = 7V means that the emitter voltage referenced to the base voltage can be 7V maximum. Often there is a "max" squeezed (UEBO,max) in to indicate it is a maximum rating. 

If you exceed that voltage the base to emitter diode will break down (avalanche). When carefully used this mode of operation can be used as a white noise generator, when uncareful the transistor will be destroyed.